feat: add download cache for offline listening

- SQLite-based index for tracking cached tracks
- Disk-based storage for audio files (~/.cache/qbz-nix/audio/)
- Download progress events via Tauri events
- LRU eviction with configurable cache size limit
- Integration with play_track for automatic local playback
- Open cache folder command for system file manager

src-tauri/Cargo.toml

@@ -22,7 +22,7 @@ serde = { version = "1", features = ["derive"] }
 serde_json = "1"
 
 # HTTP Client
-reqwest = { version = "0.12", features = ["json", "cookies", "rustls-tls"], default-features = false }
+reqwest = { version = "0.12", features = ["json", "cookies", "rustls-tls", "stream"], default-features = false }
 
 # Async Runtime
 tokio = { version = "1", features = ["full"] }

src-tauri/src/commands/playback.rs

@@ -7,20 +7,55 @@ use tokio::sync::Mutex;
 use crate::api::client::QobuzClient;
 use crate::api::models::Quality;
 use crate::cache::AudioCache;
+use crate::download_cache::DownloadCacheState;
 use crate::player::PlaybackState;
 use crate::queue::QueueManager;
 use crate::AppState;
 
 /// Play a track by ID (with caching support)
 #[tauri::command]
-pub async fn play_track(track_id: u64, state: State<'_, AppState>) -> Result<(), String> {
+pub async fn play_track(
+    track_id: u64,
+    state: State<'_, AppState>,
+    download_cache: State<'_, DownloadCacheState>,
+) -> Result<(), String> {
     log::info!("Command: play_track {}", track_id);
 
+    // First check download cache (persistent disk cache)
+    {
+        let db = download_cache.db.lock().await;
+        if let Ok(Some(file_path)) = db.get_file_path(track_id) {
+            let path = std::path::Path::new(&file_path);
+            if path.exists() {
+                log::info!("Playing track {} from download cache: {:?}", track_id, path);
+
+                // Update last accessed time
+                let _ = db.touch(track_id);
+                drop(db);  // Release lock before reading file
+
+                // Read file and play
+                let audio_data = std::fs::read(path)
+                    .map_err(|e| format!("Failed to read cached file: {}", e))?;
+
+                state.player.play_data(audio_data, track_id)?;
+
+                // Prefetch next track in background
+                spawn_prefetch(
+                    state.client.clone(),
+                    state.audio_cache.clone(),
+                    &state.queue,
+                );
+
+                return Ok(());
+            }
+        }
+    }
+
     let cache = state.audio_cache.clone();
 
-    // Check if track is in cache
+    // Check if track is in memory cache
     if let Some(cached) = cache.get(track_id) {
-        log::info!("Playing track {} from cache ({} bytes)", track_id, cached.size_bytes);
+        log::info!("Playing track {} from memory cache ({} bytes)", track_id, cached.size_bytes);
         state.player.play_data(cached.data, track_id)?;
 
         // Prefetch next track in background
@@ -33,8 +68,8 @@ pub async fn play_track(track_id: u64, state: State<'_, AppState>) -> Result<(),
         return Ok(());
     }
 
-    // Not in cache - download and cache
-    log::info!("Track {} not in cache, downloading...", track_id);
+    // Not in any cache - download and cache in memory
+    log::info!("Track {} not in cache, streaming...", track_id);
 
     let client = state.client.lock().await;
 

src-tauri/src/download_cache/commands.rs

@@ -0,0 +1,308 @@
+//! Tauri commands for download cache functionality
+
+use tauri::{AppHandle, Emitter, State};
+
+use crate::api::models::Quality;
+use crate::AppState;
+
+use super::{
+    CachedTrackInfo, DownloadCacheState, DownloadCacheStats, DownloadStatus,
+    TrackDownloadInfo,
+};
+
+/// Download a track for offline listening
+#[tauri::command]
+pub async fn download_track(
+    track_id: u64,
+    title: String,
+    artist: String,
+    album: Option<String>,
+    album_id: Option<String>,
+    duration_secs: u64,
+    quality: String,
+    bit_depth: Option<u32>,
+    sample_rate: Option<f64>,
+    state: State<'_, AppState>,
+    cache_state: State<'_, DownloadCacheState>,
+    app_handle: AppHandle,
+) -> Result<(), String> {
+    log::info!("Command: download_track {} - {} by {}", track_id, title, artist);
+
+    let track_info = TrackDownloadInfo {
+        track_id,
+        title,
+        artist,
+        album,
+        album_id,
+        duration_secs,
+        quality,
+        bit_depth,
+        sample_rate,
+    };
+
+    // Determine file path (we'll use flac as default format)
+    let file_path = cache_state.track_file_path(track_id, "flac");
+    let file_path_str = file_path.to_string_lossy().to_string();
+
+    // Insert into database as queued
+    {
+        let db = cache_state.db.lock().await;
+        db.insert_track(&track_info, &file_path_str)?;
+    }
+
+    // Emit started event
+    let _ = app_handle.emit("download:started", serde_json::json!({
+        "trackId": track_id
+    }));
+
+    // Clone what we need for the spawn
+    let client = state.client.clone();
+    let downloader = cache_state.downloader.clone();
+    let db = cache_state.db.clone();
+    let app = app_handle.clone();
+
+    // Spawn download task
+    tokio::spawn(async move {
+        // Update status to downloading
+        {
+            let db_guard = db.lock().await;
+            let _ = db_guard.update_status(track_id, DownloadStatus::Downloading, None);
+        }
+
+        // Get stream URL
+        let stream_url = {
+            let client_guard = client.lock().await;
+            client_guard
+                .get_stream_url_with_fallback(track_id, Quality::HiRes)
+                .await
+        };
+
+        let url = match stream_url {
+            Ok(s) => s.url,
+            Err(e) => {
+                log::error!("Failed to get stream URL for track {}: {}", track_id, e);
+                let db_guard = db.lock().await;
+                let _ = db_guard.update_status(
+                    track_id,
+                    DownloadStatus::Failed,
+                    Some(&format!("Failed to get stream URL: {}", e)),
+                );
+                let _ = app.emit("download:failed", serde_json::json!({
+                    "trackId": track_id,
+                    "error": e.to_string()
+                }));
+                return;
+            }
+        };
+
+        // Download the file
+        match downloader
+            .download_to_file(&url, &file_path, track_id, Some(&app))
+            .await
+        {
+            Ok(size) => {
+                log::info!("Download complete for track {}: {} bytes", track_id, size);
+                let db_guard = db.lock().await;
+                let _ = db_guard.mark_complete(track_id, size);
+                let _ = app.emit("download:completed", serde_json::json!({
+                    "trackId": track_id,
+                    "size": size
+                }));
+            }
+            Err(e) => {
+                log::error!("Download failed for track {}: {}", track_id, e);
+                let db_guard = db.lock().await;
+                let _ = db_guard.update_status(track_id, DownloadStatus::Failed, Some(&e));
+                let _ = app.emit("download:failed", serde_json::json!({
+                    "trackId": track_id,
+                    "error": e
+                }));
+            }
+        }
+    });
+
+    Ok(())
+}
+
+/// Check if a track is cached and ready for playback
+#[tauri::command]
+pub async fn is_track_downloaded(
+    track_id: u64,
+    cache_state: State<'_, DownloadCacheState>,
+) -> Result<bool, String> {
+    let db = cache_state.db.lock().await;
+    db.is_cached(track_id)
+}
+
+/// Get the local file path for a cached track
+#[tauri::command]
+pub async fn get_downloaded_track_path(
+    track_id: u64,
+    cache_state: State<'_, DownloadCacheState>,
+) -> Result<Option<String>, String> {
+    let db = cache_state.db.lock().await;
+    db.get_file_path(track_id)
+}
+
+/// Get info about a specific cached track
+#[tauri::command]
+pub async fn get_downloaded_track(
+    track_id: u64,
+    cache_state: State<'_, DownloadCacheState>,
+) -> Result<Option<CachedTrackInfo>, String> {
+    let db = cache_state.db.lock().await;
+    db.get_track(track_id)
+}
+
+/// Get all cached tracks
+#[tauri::command]
+pub async fn get_downloaded_tracks(
+    cache_state: State<'_, DownloadCacheState>,
+) -> Result<Vec<CachedTrackInfo>, String> {
+    let db = cache_state.db.lock().await;
+    db.get_all_tracks()
+}
+
+/// Get download cache statistics
+#[tauri::command]
+pub async fn get_download_cache_stats(
+    cache_state: State<'_, DownloadCacheState>,
+) -> Result<DownloadCacheStats, String> {
+    let db = cache_state.db.lock().await;
+    let limit = *cache_state.limit_bytes.lock().await;
+    db.get_stats(&cache_state.get_cache_path(), limit)
+}
+
+/// Remove a track from the download cache
+#[tauri::command]
+pub async fn remove_downloaded_track(
+    track_id: u64,
+    cache_state: State<'_, DownloadCacheState>,
+) -> Result<(), String> {
+    log::info!("Command: remove_downloaded_track {}", track_id);
+
+    let db = cache_state.db.lock().await;
+
+    // Get file path and delete from DB
+    if let Some(file_path) = db.delete_track(track_id)? {
+        // Delete the actual file
+        let path = std::path::Path::new(&file_path);
+        if path.exists() {
+            std::fs::remove_file(path)
+                .map_err(|e| format!("Failed to delete file: {}", e))?;
+        }
+    }
+
+    Ok(())
+}
+
+/// Clear entire download cache
+#[tauri::command]
+pub async fn clear_download_cache(
+    cache_state: State<'_, DownloadCacheState>,
+) -> Result<(), String> {
+    log::info!("Command: clear_download_cache");
+
+    let db = cache_state.db.lock().await;
+
+    // Get all file paths and clear DB
+    let paths = db.clear_all()?;
+
+    // Delete all files
+    for path in paths {
+        let p = std::path::Path::new(&path);
+        if p.exists() {
+            let _ = std::fs::remove_file(p);
+        }
+    }
+
+    // Also clear the tracks directory
+    let tracks_dir = cache_state.cache_dir.join("tracks");
+    if tracks_dir.exists() {
+        if let Ok(entries) = std::fs::read_dir(&tracks_dir) {
+            for entry in entries.flatten() {
+                let _ = std::fs::remove_file(entry.path());
+            }
+        }
+    }
+
+    Ok(())
+}
+
+/// Set cache size limit
+#[tauri::command]
+pub async fn set_download_cache_limit(
+    limit_mb: Option<u64>,
+    cache_state: State<'_, DownloadCacheState>,
+) -> Result<(), String> {
+    log::info!("Command: set_download_cache_limit {:?} MB", limit_mb);
+
+    let limit_bytes = limit_mb.map(|mb| mb * 1024 * 1024);
+    let mut limit = cache_state.limit_bytes.lock().await;
+    *limit = limit_bytes;
+
+    // Check if we need to evict
+    if let Some(limit_bytes) = limit_bytes {
+        drop(limit);
+        evict_if_needed(&cache_state, limit_bytes).await?;
+    }
+
+    Ok(())
+}
+
+/// Open the cache folder in the system file manager
+#[tauri::command]
+pub async fn open_download_cache_folder(
+    cache_state: State<'_, DownloadCacheState>,
+) -> Result<(), String> {
+    log::info!("Command: open_download_cache_folder");
+
+    let path = cache_state.cache_dir.clone();
+
+    // Ensure directory exists
+    std::fs::create_dir_all(&path)
+        .map_err(|e| format!("Failed to create cache directory: {}", e))?;
+
+    // Open with system file manager
+    open::that(&path).map_err(|e| format!("Failed to open folder: {}", e))?;
+
+    Ok(())
+}
+
+/// Evict tracks if cache exceeds limit (LRU policy)
+async fn evict_if_needed(
+    cache_state: &DownloadCacheState,
+    limit_bytes: u64,
+) -> Result<(), String> {
+    let db = cache_state.db.lock().await;
+    let stats = db.get_stats(&cache_state.get_cache_path(), Some(limit_bytes))?;
+
+    if stats.total_size_bytes <= limit_bytes {
+        return Ok(());
+    }
+
+    let bytes_to_free = stats.total_size_bytes - limit_bytes;
+    log::info!(
+        "Cache exceeds limit ({} > {}), need to free {} bytes",
+        stats.total_size_bytes,
+        limit_bytes,
+        bytes_to_free
+    );
+
+    let tracks_to_evict = db.get_tracks_for_eviction(bytes_to_free)?;
+
+    for (track_id, file_path) in tracks_to_evict {
+        log::info!("Evicting track {} from cache", track_id);
+
+        // Delete from DB
+        db.delete_track(track_id)?;
+
+        // Delete file
+        let path = std::path::Path::new(&file_path);
+        if path.exists() {
+            let _ = std::fs::remove_file(path);
+        }
+    }
+
+    Ok(())
+}

src-tauri/src/download_cache/db.rs

@@ -0,0 +1,324 @@
+//! SQLite database for download cache index
+
+use rusqlite::{Connection, params};
+use std::path::Path;
+
+use super::{CachedTrackInfo, DownloadCacheStats, DownloadStatus, TrackDownloadInfo};
+
+/// Database wrapper for cached tracks index
+pub struct DownloadCacheDb {
+    conn: Connection,
+}
+
+impl DownloadCacheDb {
+    /// Open or create the database
+    pub fn new(path: &Path) -> Result<Self, String> {
+        let conn = Connection::open(path)
+            .map_err(|e| format!("Failed to open download cache database: {}", e))?;
+
+        let db = Self { conn };
+        db.init_schema()?;
+        Ok(db)
+    }
+
+    /// Initialize database schema
+    fn init_schema(&self) -> Result<(), String> {
+        self.conn.execute_batch(
+            "
+            CREATE TABLE IF NOT EXISTS cached_tracks (
+                id INTEGER PRIMARY KEY AUTOINCREMENT,
+                track_id INTEGER UNIQUE NOT NULL,
+                title TEXT NOT NULL,
+                artist TEXT NOT NULL,
+                album TEXT,
+                album_id TEXT,
+                duration_secs INTEGER NOT NULL,
+                file_path TEXT NOT NULL,
+                file_size_bytes INTEGER NOT NULL DEFAULT 0,
+                format TEXT NOT NULL DEFAULT 'flac',
+                quality TEXT,
+                bit_depth INTEGER,
+                sample_rate REAL,
+                artwork_path TEXT,
+                status TEXT NOT NULL DEFAULT 'queued',
+                progress_percent INTEGER NOT NULL DEFAULT 0,
+                error_message TEXT,
+                created_at TEXT NOT NULL DEFAULT (datetime('now')),
+                last_accessed_at TEXT NOT NULL DEFAULT (datetime('now'))
+            );
+
+            CREATE INDEX IF NOT EXISTS idx_track_id ON cached_tracks(track_id);
+            CREATE INDEX IF NOT EXISTS idx_status ON cached_tracks(status);
+            CREATE INDEX IF NOT EXISTS idx_last_accessed ON cached_tracks(last_accessed_at);
+            "
+        ).map_err(|e| format!("Failed to initialize database schema: {}", e))?;
+
+        Ok(())
+    }
+
+    /// Insert a new track to download
+    pub fn insert_track(&self, info: &TrackDownloadInfo, file_path: &str) -> Result<(), String> {
+        self.conn.execute(
+            "INSERT OR REPLACE INTO cached_tracks
+             (track_id, title, artist, album, album_id, duration_secs, file_path, quality, bit_depth, sample_rate, status, progress_percent, created_at, last_accessed_at)
+             VALUES (?1, ?2, ?3, ?4, ?5, ?6, ?7, ?8, ?9, ?10, 'queued', 0, datetime('now'), datetime('now'))",
+            params![
+                info.track_id as i64,
+                info.title,
+                info.artist,
+                info.album,
+                info.album_id,
+                info.duration_secs as i64,
+                file_path,
+                info.quality,
+                info.bit_depth.map(|v| v as i64),
+                info.sample_rate,
+            ],
+        ).map_err(|e| format!("Failed to insert track: {}", e))?;
+
+        Ok(())
+    }
+
+    /// Update track status
+    pub fn update_status(&self, track_id: u64, status: DownloadStatus, error: Option<&str>) -> Result<(), String> {
+        self.conn.execute(
+            "UPDATE cached_tracks SET status = ?1, error_message = ?2 WHERE track_id = ?3",
+            params![status.as_str(), error, track_id as i64],
+        ).map_err(|e| format!("Failed to update status: {}", e))?;
+
+        Ok(())
+    }
+
+    /// Update download progress
+    pub fn update_progress(&self, track_id: u64, progress: u8, size_bytes: u64) -> Result<(), String> {
+        self.conn.execute(
+            "UPDATE cached_tracks SET progress_percent = ?1, file_size_bytes = ?2 WHERE track_id = ?3",
+            params![progress as i64, size_bytes as i64, track_id as i64],
+        ).map_err(|e| format!("Failed to update progress: {}", e))?;
+
+        Ok(())
+    }
+
+    /// Mark download as complete
+    pub fn mark_complete(&self, track_id: u64, file_size: u64) -> Result<(), String> {
+        self.conn.execute(
+            "UPDATE cached_tracks SET status = 'ready', progress_percent = 100, file_size_bytes = ?1, last_accessed_at = datetime('now') WHERE track_id = ?2",
+            params![file_size as i64, track_id as i64],
+        ).map_err(|e| format!("Failed to mark complete: {}", e))?;
+
+        Ok(())
+    }
+
+    /// Update last accessed time (for LRU)
+    pub fn touch(&self, track_id: u64) -> Result<(), String> {
+        self.conn.execute(
+            "UPDATE cached_tracks SET last_accessed_at = datetime('now') WHERE track_id = ?1",
+            params![track_id as i64],
+        ).map_err(|e| format!("Failed to update access time: {}", e))?;
+
+        Ok(())
+    }
+
+    /// Check if a track is cached and ready
+    pub fn is_cached(&self, track_id: u64) -> Result<bool, String> {
+        let count: i64 = self.conn.query_row(
+            "SELECT COUNT(*) FROM cached_tracks WHERE track_id = ?1 AND status = 'ready'",
+            params![track_id as i64],
+            |row| row.get(0),
+        ).map_err(|e| format!("Failed to check cache: {}", e))?;
+
+        Ok(count > 0)
+    }
+
+    /// Get file path for a cached track
+    pub fn get_file_path(&self, track_id: u64) -> Result<Option<String>, String> {
+        let result = self.conn.query_row(
+            "SELECT file_path FROM cached_tracks WHERE track_id = ?1 AND status = 'ready'",
+            params![track_id as i64],
+            |row| row.get(0),
+        );
+
+        match result {
+            Ok(path) => Ok(Some(path)),
+            Err(rusqlite::Error::QueryReturnedNoRows) => Ok(None),
+            Err(e) => Err(format!("Failed to get file path: {}", e)),
+        }
+    }
+
+    /// Get track info
+    pub fn get_track(&self, track_id: u64) -> Result<Option<CachedTrackInfo>, String> {
+        let result = self.conn.query_row(
+            "SELECT track_id, title, artist, album, album_id, duration_secs, file_size_bytes, quality, bit_depth, sample_rate, status, progress_percent, error_message, created_at, last_accessed_at
+             FROM cached_tracks WHERE track_id = ?1",
+            params![track_id as i64],
+            |row| {
+                Ok(CachedTrackInfo {
+                    track_id: row.get::<_, i64>(0)? as u64,
+                    title: row.get(1)?,
+                    artist: row.get(2)?,
+                    album: row.get(3)?,
+                    album_id: row.get(4)?,
+                    duration_secs: row.get::<_, i64>(5)? as u64,
+                    file_size_bytes: row.get::<_, i64>(6)? as u64,
+                    quality: row.get::<_, Option<String>>(7)?.unwrap_or_default(),
+                    bit_depth: row.get::<_, Option<i64>>(8)?.map(|v| v as u32),
+                    sample_rate: row.get(9)?,
+                    status: DownloadStatus::from_str(&row.get::<_, String>(10)?),
+                    progress_percent: row.get::<_, i64>(11)? as u8,
+                    error_message: row.get(12)?,
+                    created_at: row.get(13)?,
+                    last_accessed_at: row.get(14)?,
+                })
+            },
+        );
+
+        match result {
+            Ok(info) => Ok(Some(info)),
+            Err(rusqlite::Error::QueryReturnedNoRows) => Ok(None),
+            Err(e) => Err(format!("Failed to get track: {}", e)),
+        }
+    }
+
+    /// Get all cached tracks
+    pub fn get_all_tracks(&self) -> Result<Vec<CachedTrackInfo>, String> {
+        let mut stmt = self.conn.prepare(
+            "SELECT track_id, title, artist, album, album_id, duration_secs, file_size_bytes, quality, bit_depth, sample_rate, status, progress_percent, error_message, created_at, last_accessed_at
+             FROM cached_tracks ORDER BY last_accessed_at DESC"
+        ).map_err(|e| format!("Failed to prepare query: {}", e))?;
+
+        let tracks = stmt.query_map([], |row| {
+            Ok(CachedTrackInfo {
+                track_id: row.get::<_, i64>(0)? as u64,
+                title: row.get(1)?,
+                artist: row.get(2)?,
+                album: row.get(3)?,
+                album_id: row.get(4)?,
+                duration_secs: row.get::<_, i64>(5)? as u64,
+                file_size_bytes: row.get::<_, i64>(6)? as u64,
+                quality: row.get::<_, Option<String>>(7)?.unwrap_or_default(),
+                bit_depth: row.get::<_, Option<i64>>(8)?.map(|v| v as u32),
+                sample_rate: row.get(9)?,
+                status: DownloadStatus::from_str(&row.get::<_, String>(10)?),
+                progress_percent: row.get::<_, i64>(11)? as u8,
+                error_message: row.get(12)?,
+                created_at: row.get(13)?,
+                last_accessed_at: row.get(14)?,
+            })
+        }).map_err(|e| format!("Failed to query tracks: {}", e))?;
+
+        let mut result = Vec::new();
+        for track in tracks {
+            result.push(track.map_err(|e| format!("Failed to read track: {}", e))?);
+        }
+
+        Ok(result)
+    }
+
+    /// Delete a track from cache
+    pub fn delete_track(&self, track_id: u64) -> Result<Option<String>, String> {
+        // Get file path before deleting
+        let file_path: Option<String> = self.conn.query_row(
+            "SELECT file_path FROM cached_tracks WHERE track_id = ?1",
+            params![track_id as i64],
+            |row| row.get(0),
+        ).ok();
+
+        self.conn.execute(
+            "DELETE FROM cached_tracks WHERE track_id = ?1",
+            params![track_id as i64],
+        ).map_err(|e| format!("Failed to delete track: {}", e))?;
+
+        Ok(file_path)
+    }
+
+    /// Get statistics
+    pub fn get_stats(&self, cache_path: &str, limit_bytes: Option<u64>) -> Result<DownloadCacheStats, String> {
+        let total_tracks: i64 = self.conn.query_row(
+            "SELECT COUNT(*) FROM cached_tracks",
+            [],
+            |row| row.get(0),
+        ).map_err(|e| format!("Failed to count tracks: {}", e))?;
+
+        let ready_tracks: i64 = self.conn.query_row(
+            "SELECT COUNT(*) FROM cached_tracks WHERE status = 'ready'",
+            [],
+            |row| row.get(0),
+        ).map_err(|e| format!("Failed to count ready tracks: {}", e))?;
+
+        let downloading_tracks: i64 = self.conn.query_row(
+            "SELECT COUNT(*) FROM cached_tracks WHERE status = 'downloading' OR status = 'queued'",
+            [],
+            |row| row.get(0),
+        ).map_err(|e| format!("Failed to count downloading tracks: {}", e))?;
+
+        let failed_tracks: i64 = self.conn.query_row(
+            "SELECT COUNT(*) FROM cached_tracks WHERE status = 'failed'",
+            [],
+            |row| row.get(0),
+        ).map_err(|e| format!("Failed to count failed tracks: {}", e))?;
+
+        let total_size: i64 = self.conn.query_row(
+            "SELECT COALESCE(SUM(file_size_bytes), 0) FROM cached_tracks WHERE status = 'ready'",
+            [],
+            |row| row.get(0),
+        ).map_err(|e| format!("Failed to sum sizes: {}", e))?;
+
+        Ok(DownloadCacheStats {
+            total_tracks: total_tracks as usize,
+            ready_tracks: ready_tracks as usize,
+            downloading_tracks: downloading_tracks as usize,
+            failed_tracks: failed_tracks as usize,
+            total_size_bytes: total_size as u64,
+            limit_bytes,
+            cache_path: cache_path.to_string(),
+        })
+    }
+
+    /// Get tracks to evict (LRU order) to free up space
+    pub fn get_tracks_for_eviction(&self, bytes_to_free: u64) -> Result<Vec<(u64, String)>, String> {
+        let mut stmt = self.conn.prepare(
+            "SELECT track_id, file_path, file_size_bytes FROM cached_tracks
+             WHERE status = 'ready'
+             ORDER BY last_accessed_at ASC"
+        ).map_err(|e| format!("Failed to prepare eviction query: {}", e))?;
+
+        let mut result = Vec::new();
+        let mut freed = 0u64;
+
+        let rows = stmt.query_map([], |row| {
+            Ok((
+                row.get::<_, i64>(0)? as u64,
+                row.get::<_, String>(1)?,
+                row.get::<_, i64>(2)? as u64,
+            ))
+        }).map_err(|e| format!("Failed to query for eviction: {}", e))?;
+
+        for row in rows {
+            if freed >= bytes_to_free {
+                break;
+            }
+            let (track_id, file_path, size) = row.map_err(|e| format!("Failed to read row: {}", e))?;
+            result.push((track_id, file_path));
+            freed += size;
+        }
+
+        Ok(result)
+    }
+
+    /// Clear all entries
+    pub fn clear_all(&self) -> Result<Vec<String>, String> {
+        // Get all file paths first
+        let mut stmt = self.conn.prepare("SELECT file_path FROM cached_tracks")
+            .map_err(|e| format!("Failed to prepare query: {}", e))?;
+
+        let paths: Vec<String> = stmt.query_map([], |row| row.get(0))
+            .map_err(|e| format!("Failed to query paths: {}", e))?
+            .filter_map(|r| r.ok())
+            .collect();
+
+        self.conn.execute("DELETE FROM cached_tracks", [])
+            .map_err(|e| format!("Failed to clear database: {}", e))?;
+
+        Ok(paths)
+    }
+}

src-tauri/src/download_cache/downloader.rs

@@ -0,0 +1,163 @@
+//! Download logic for caching tracks to disk
+
+use std::io::Write;
+use std::path::Path;
+use std::time::Duration;
+use tauri::{AppHandle, Emitter};
+
+use super::{DownloadProgress, DownloadStatus};
+
+/// Downloader handles fetching audio files and saving them to disk
+pub struct Downloader {
+    client: reqwest::Client,
+}
+
+impl Downloader {
+    pub fn new() -> Self {
+        let client = reqwest::Client::builder()
+            .timeout(Duration::from_secs(300)) // 5 minute timeout for large files
+            .connect_timeout(Duration::from_secs(15))
+            .build()
+            .expect("Failed to create HTTP client");
+
+        Self { client }
+    }
+
+    /// Download a file to disk with progress updates
+    pub async fn download_to_file(
+        &self,
+        url: &str,
+        dest_path: &Path,
+        track_id: u64,
+        app_handle: Option<&AppHandle>,
+    ) -> Result<u64, String> {
+        log::info!("Downloading track {} to {:?}", track_id, dest_path);
+
+        // Create parent directories if needed
+        if let Some(parent) = dest_path.parent() {
+            std::fs::create_dir_all(parent)
+                .map_err(|e| format!("Failed to create directory: {}", e))?;
+        }
+
+        // Start the download
+        let response = self
+            .client
+            .get(url)
+            .header("User-Agent", "Mozilla/5.0")
+            .send()
+            .await
+            .map_err(|e| format!("Failed to start download: {}", e))?;
+
+        if !response.status().is_success() {
+            return Err(format!("HTTP error: {}", response.status()));
+        }
+
+        let total_size = response.content_length();
+        log::info!(
+            "Download started for track {}, total size: {:?} bytes",
+            track_id,
+            total_size
+        );
+
+        // Create temp file for downloading
+        let temp_path = dest_path.with_extension("tmp");
+        let mut file = std::fs::File::create(&temp_path)
+            .map_err(|e| format!("Failed to create temp file: {}", e))?;
+
+        let mut downloaded: u64 = 0;
+        let mut last_progress: u8 = 0;
+
+        // Stream the response body
+        let mut stream = response.bytes_stream();
+        use futures_util::StreamExt;
+
+        while let Some(chunk_result) = stream.next().await {
+            let chunk = chunk_result.map_err(|e| format!("Download error: {}", e))?;
+
+            file.write_all(&chunk)
+                .map_err(|e| format!("Failed to write chunk: {}", e))?;
+
+            downloaded += chunk.len() as u64;
+
+            // Calculate progress
+            let progress = if let Some(total) = total_size {
+                ((downloaded as f64 / total as f64) * 100.0) as u8
+            } else {
+                // If we don't know total size, report bytes downloaded
+                0
+            };
+
+            // Emit progress event every 2% change
+            if progress != last_progress && (progress - last_progress >= 2 || progress == 100) {
+                last_progress = progress;
+
+                if let Some(app) = app_handle {
+                    let _ = app.emit(
+                        "download:progress",
+                        DownloadProgress {
+                            track_id,
+                            progress_percent: progress,
+                            bytes_downloaded: downloaded,
+                            total_bytes: total_size,
+                            status: DownloadStatus::Downloading,
+                        },
+                    );
+                }
+
+                log::debug!(
+                    "Download progress for track {}: {}% ({}/{:?} bytes)",
+                    track_id,
+                    progress,
+                    downloaded,
+                    total_size
+                );
+            }
+        }
+
+        // Ensure all data is written
+        file.flush()
+            .map_err(|e| format!("Failed to flush file: {}", e))?;
+        drop(file);
+
+        // Move temp file to final destination
+        std::fs::rename(&temp_path, dest_path)
+            .map_err(|e| format!("Failed to move temp file: {}", e))?;
+
+        log::info!(
+            "Download complete for track {}: {} bytes",
+            track_id,
+            downloaded
+        );
+
+        Ok(downloaded)
+    }
+
+    /// Download to memory (for smaller files or streaming)
+    pub async fn download_to_memory(&self, url: &str) -> Result<Vec<u8>, String> {
+        let response = self
+            .client
+            .get(url)
+            .header("User-Agent", "Mozilla/5.0")
+            .send()
+            .await
+            .map_err(|e| format!("Failed to fetch: {}", e))?;
+
+        if !response.status().is_success() {
+            return Err(format!("HTTP error: {}", response.status()));
+        }
+
+        let bytes = response
+            .bytes()
+            .await
+            .map_err(|e| format!("Failed to read bytes: {}", e))?;
+
+        Ok(bytes.to_vec())
+    }
+}
+
+impl Default for Downloader {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+

src-tauri/src/download_cache/mod.rs

@@ -0,0 +1,168 @@
+//! Download cache module for offline listening
+//!
+//! Provides persistent disk-based caching for audio tracks:
+//! - SQLite index for track metadata
+//! - File-based storage for audio data
+//! - LRU eviction with configurable limits
+//! - Progress events for UI updates
+
+pub mod commands;
+pub mod db;
+pub mod downloader;
+
+use std::path::PathBuf;
+use std::sync::Arc;
+use tokio::sync::Mutex;
+use serde::{Deserialize, Serialize};
+
+pub use db::DownloadCacheDb;
+pub use downloader::Downloader;
+
+/// Download status for a cached track
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
+#[serde(rename_all = "lowercase")]
+pub enum DownloadStatus {
+    Queued,
+    Downloading,
+    Ready,
+    Failed,
+}
+
+impl DownloadStatus {
+    pub fn as_str(&self) -> &'static str {
+        match self {
+            Self::Queued => "queued",
+            Self::Downloading => "downloading",
+            Self::Ready => "ready",
+            Self::Failed => "failed",
+        }
+    }
+
+    pub fn from_str(s: &str) -> Self {
+        match s {
+            "queued" => Self::Queued,
+            "downloading" => Self::Downloading,
+            "ready" => Self::Ready,
+            "failed" => Self::Failed,
+            _ => Self::Failed,
+        }
+    }
+}
+
+/// Information about a cached track
+#[derive(Debug, Clone, Serialize)]
+#[serde(rename_all = "camelCase")]
+pub struct CachedTrackInfo {
+    pub track_id: u64,
+    pub title: String,
+    pub artist: String,
+    pub album: Option<String>,
+    pub album_id: Option<String>,
+    pub duration_secs: u64,
+    pub file_size_bytes: u64,
+    pub quality: String,
+    pub bit_depth: Option<u32>,
+    pub sample_rate: Option<f64>,
+    pub status: DownloadStatus,
+    pub progress_percent: u8,
+    pub error_message: Option<String>,
+    pub created_at: String,
+    pub last_accessed_at: String,
+}
+
+/// Statistics about the download cache
+#[derive(Debug, Clone, Serialize)]
+#[serde(rename_all = "camelCase")]
+pub struct DownloadCacheStats {
+    pub total_tracks: usize,
+    pub ready_tracks: usize,
+    pub downloading_tracks: usize,
+    pub failed_tracks: usize,
+    pub total_size_bytes: u64,
+    pub limit_bytes: Option<u64>,
+    pub cache_path: String,
+}
+
+/// Progress update for a download
+#[derive(Debug, Clone, Serialize)]
+#[serde(rename_all = "camelCase")]
+pub struct DownloadProgress {
+    pub track_id: u64,
+    pub progress_percent: u8,
+    pub bytes_downloaded: u64,
+    pub total_bytes: Option<u64>,
+    pub status: DownloadStatus,
+}
+
+/// Track metadata for initiating a download
+#[derive(Debug, Clone)]
+pub struct TrackDownloadInfo {
+    pub track_id: u64,
+    pub title: String,
+    pub artist: String,
+    pub album: Option<String>,
+    pub album_id: Option<String>,
+    pub duration_secs: u64,
+    pub quality: String,
+    pub bit_depth: Option<u32>,
+    pub sample_rate: Option<f64>,
+}
+
+/// Download cache state manager
+pub struct DownloadCacheState {
+    pub db: Arc<Mutex<DownloadCacheDb>>,
+    pub downloader: Arc<Downloader>,
+    pub cache_dir: PathBuf,
+    /// Cache limit in bytes (None = unlimited)
+    pub limit_bytes: Arc<Mutex<Option<u64>>>,
+}
+
+impl DownloadCacheState {
+    /// Initialize the download cache
+    pub fn new() -> Result<Self, String> {
+        let cache_dir = dirs::cache_dir()
+            .ok_or("Could not determine cache directory")?
+            .join("qbz-nix")
+            .join("audio");
+
+        // Create directories
+        std::fs::create_dir_all(&cache_dir)
+            .map_err(|e| format!("Failed to create cache directory: {}", e))?;
+        std::fs::create_dir_all(cache_dir.join("tracks"))
+            .map_err(|e| format!("Failed to create tracks directory: {}", e))?;
+        std::fs::create_dir_all(cache_dir.join("artwork"))
+            .map_err(|e| format!("Failed to create artwork directory: {}", e))?;
+
+        let db_path = cache_dir.join("index.db");
+        let db = DownloadCacheDb::new(&db_path)?;
+
+        // Default limit: 2GB
+        let default_limit = Some(2 * 1024 * 1024 * 1024u64);
+
+        let state = Self {
+            db: Arc::new(Mutex::new(db)),
+            downloader: Arc::new(Downloader::new()),
+            cache_dir: cache_dir.clone(),
+            limit_bytes: Arc::new(Mutex::new(default_limit)),
+        };
+
+        log::info!("Download cache initialized at: {:?}", cache_dir);
+
+        Ok(state)
+    }
+
+    /// Get the path for a track's audio file
+    pub fn track_file_path(&self, track_id: u64, format: &str) -> PathBuf {
+        self.cache_dir.join("tracks").join(format!("{}.{}", track_id, format))
+    }
+
+    /// Get the path for an album's artwork
+    pub fn artwork_path(&self, album_id: &str) -> PathBuf {
+        self.cache_dir.join("artwork").join(format!("{}.jpg", album_id))
+    }
+
+    /// Get the cache directory path
+    pub fn get_cache_path(&self) -> String {
+        self.cache_dir.to_string_lossy().to_string()
+    }
+}

src-tauri/src/lib.rs

@@ -10,6 +10,7 @@ pub mod commands;
 pub mod config;
 pub mod credentials;
 pub mod discogs;
+pub mod download_cache;
 pub mod lastfm;
 pub mod library;
 pub mod media_controls;
@@ -99,6 +100,10 @@ pub fn run() {
     let cast_state = cast::CastState::new()
         .expect("Failed to initialize casting state");
 
+    // Initialize download cache state
+    let download_cache_state = download_cache::DownloadCacheState::new()
+        .expect("Failed to initialize download cache");
+
     tauri::Builder::default()
         .plugin(tauri_plugin_opener::init())
         .plugin(tauri_plugin_dialog::init())
@@ -106,6 +111,7 @@ pub fn run() {
         .manage(AppState::new())
         .manage(library_state)
         .manage(cast_state)
+        .manage(download_cache_state)
         .invoke_handler(tauri::generate_handler![
             // Auth commands
             commands::init_client,
@@ -230,6 +236,17 @@ pub fn run() {
             cast::commands::cast_stop,
             cast::commands::cast_seek,
             cast::commands::cast_set_volume,
+            // Download cache commands
+            download_cache::commands::download_track,
+            download_cache::commands::is_track_downloaded,
+            download_cache::commands::get_downloaded_track_path,
+            download_cache::commands::get_downloaded_track,
+            download_cache::commands::get_downloaded_tracks,
+            download_cache::commands::get_download_cache_stats,
+            download_cache::commands::remove_downloaded_track,
+            download_cache::commands::clear_download_cache,
+            download_cache::commands::set_download_cache_limit,
+            download_cache::commands::open_download_cache_folder,
         ])
         .run(tauri::generate_context!())
         .expect("error while running tauri application");