fix(local-library): use high-positive ids for ephemeral tracks (queue/context u64 compat)

The original design used negative i64 ids to distinguish ephemeral
tracks from DB rows, but the queue and playback-context commands
serialize ids as Vec<u64> end-to-end (V2QueueTrack, v2_set_playback_context).
serde rejects negative numbers at the boundary, so setQueueForLocalTracks
and setPlaybackContext failed the moment the user clicked an ephemeral
track:

    invalid value: integer `-1`, expected u64

Switch synthetic ids to a high-positive range (EPHEMERAL_ID_FLOOR =
1<<48) instead. Pros over the negative scheme:
* Valid u64 — passes through the queue/context commands untouched
* Below 2^53 so JS Number round-trips without precision loss
* Far above any realistic DB autoincrement id, so the discrimination
  test (`track_id >= EPHEMERAL_ID_FLOOR`) stays unambiguous

Also fixes the snake_case mismatch on EphemeralFolderState — the Rust
struct serializes with its native field names (folder_path,
skipped_files), but the TypeScript type was declared in camelCase, so
ephemeralFolder.folderPath was undefined at render time.

cc

src-tauri/src/commands_v2/library.rs

@@ -839,12 +839,15 @@ pub async fn v2_library_play_track(
         .await
         .map_err(|e| e.to_string())?;
 
-    // Negative ids belong to the ephemeral library (an ad-hoc folder the
-    // user opened without persisting it to local_tracks). Resolve to the
-    // in-memory cache and skip every DB-bound branch below — ephemeral
-    // tracks are never qobuz_download, never offline-cached, never have a
-    // CUE pointer. Just read the file and hand it to the player.
-    if track_id < 0 {
+    // Track ids at or above EPHEMERAL_ID_FLOOR belong to the ephemeral
+    // library (an ad-hoc folder the user opened without persisting it
+    // to local_tracks). Resolve to the in-memory cache and skip every
+    // DB-bound branch below — ephemeral tracks are never qobuz_download,
+    // never offline-cached, never have a CUE pointer. Just read the
+    // file and hand it to the player. The high-positive id range keeps
+    // the existing u64-typed queue/context commands happy while still
+    // being unambiguous (DB autoincrement ids never reach 2^48).
+    if track_id >= crate::ephemeral_library::EPHEMERAL_ID_FLOOR {
         let track = ephemeral_state
             .get_track(track_id)
             .ok_or_else(|| format!("Ephemeral track {} not found (session may have been cleared)", track_id))?;

src-tauri/src/ephemeral_library/mod.rs

@@ -4,12 +4,20 @@
 //! (a downloaded album they haven't decided to keep, an external drive,
 //! etc.), browse it, and play tracks from it without anything landing
 //! in `local_tracks`. The ephemeral session lives only in memory: a
-//! `HashMap<i64, LocalTrack>` keyed by *synthetic negative ids*. Negative
-//! ids are how the rest of the playback pipeline distinguishes ephemeral
-//! tracks from DB-resolvable ones — DB ids are always positive
-//! (autoincrement), so a negative `track_id` arriving at
-//! `v2_library_play_track` is unambiguously an ephemeral track and gets
-//! routed here instead of the DB.
+//! `HashMap<i64, LocalTrack>` keyed by *synthetic ids in the high
+//! range* (>= `EPHEMERAL_ID_FLOOR` = 2^48). Synthetic ids in this range
+//! are how the rest of the playback pipeline distinguishes ephemeral
+//! tracks from DB-resolvable ones — local_tracks autoincrement IDs are
+//! orders of magnitude smaller, so any track_id arriving at
+//! `v2_library_play_track` at or above the floor is unambiguously
+//! ephemeral and gets routed here instead of the DB.
+//!
+//! The high-positive design (instead of the obvious "use negatives")
+//! exists because the queue/playback-context commands serialize ids as
+//! `u64` end-to-end (V2QueueTrack, v2_set_playback_context) and reject
+//! negative numbers at the serde boundary. Positive ids above the DB
+//! range and below 2^53 (JS Number safe limit) are valid u64 *and*
+//! survive the JSON round-trip without precision loss.
 //!
 //! Only one folder is held at a time; opening a new folder replaces the
 //! previous session. The state vanishes on app exit by virtue of being
@@ -22,6 +30,13 @@ use std::sync::Mutex;
 use qbz_library::{LibraryError, LibraryScanner, LocalTrack, MetadataExtractor};
 use serde::Serialize;
 
+/// Floor for synthetic ephemeral track ids. Any id at or above this
+/// value is an ephemeral track; below it is a DB row id. Set high
+/// enough to be impossible to collide with autoincrement DB ids in any
+/// realistic library size, low enough to fit in JS Number's safe
+/// integer range (2^53 - 1) so the JSON round-trip stays lossless.
+pub const EPHEMERAL_ID_FLOOR: i64 = 1 << 48;
+
 #[derive(Debug, Serialize, Clone)]
 pub struct EphemeralFolderResult {
     pub folder_path: String,
@@ -62,14 +77,14 @@ impl EphemeralLibraryInner {
     fn new() -> Self {
         Self {
             tracks: HashMap::new(),
-            next_id: -1,
+            next_id: EPHEMERAL_ID_FLOOR,
             current_folder_path: None,
         }
     }
 
     fn reset(&mut self) {
         self.tracks.clear();
-        self.next_id = -1;
+        self.next_id = EPHEMERAL_ID_FLOOR;
         self.current_folder_path = None;
     }
 }
@@ -115,7 +130,7 @@ impl EphemeralLibraryState {
             match MetadataExtractor::extract(audio_file) {
                 Ok(mut track) => {
                     track.id = inner.next_id;
-                    inner.next_id -= 1;
+                    inner.next_id += 1;
                     track.source = Some("ephemeral".to_string());
                     inner.tracks.insert(track.id, track.clone());
                     tracks_out.push(track);

src/lib/components/views/LocalLibraryView.svelte

@@ -600,10 +600,10 @@
       ephemeralFolder = result;
       if (result.tracks.length === 0) {
         showToast($t('library.ephemeralFolder.empty'), 'info');
-      } else if (result.skippedFiles > 0) {
+      } else if (result.skipped_files > 0) {
         showToast(
           $t('library.ephemeralFolder.openedWithSkips', {
-            values: { count: result.tracks.length, skipped: result.skippedFiles }
+            values: { count: result.tracks.length, skipped: result.skipped_files }
           }),
           'success'
         );
@@ -1009,18 +1009,20 @@
   // tracks here have synthetic negative ids that route to the in-memory
   // EphemeralLibraryState on the backend (see v2_library_play_track). The
   // session vanishes on app exit (no DB persistence).
+  // Field names are snake_case to match the Rust struct serde-serializes
+  // (consistent with LocalTrack, which is also snake_case across the FFI).
   type EphemeralFolderState = {
-    folderPath: string;
+    folder_path: string;
     tracks: LocalTrack[];
-    skippedFiles: number;
+    skipped_files: number;
   };
   let ephemeralFolder = $state<EphemeralFolderState | null>(null);
   let openingEphemeralFolder = $state(false);
 
   const ephemeralFolderName = $derived.by(() => {
     if (!ephemeralFolder) return '';
-    const segments = ephemeralFolder.folderPath.split('/').filter(Boolean);
-    return segments.length > 0 ? segments[segments.length - 1] : ephemeralFolder.folderPath;
+    const segments = ephemeralFolder.folder_path.split('/').filter(Boolean);
+    return segments.length > 0 ? segments[segments.length - 1] : ephemeralFolder.folder_path;
   });
   // Set of paths the user has expanded in the tree. Each top-level
   // <LocalLibraryFolderTree> shares this set so siblings stay in sync.
@@ -5707,7 +5709,7 @@
                             {$t('library.ephemeralFolder.subtitle', {
                               values: {
                                 count: ephemeralFolder.tracks.length,
-                                path: ephemeralFolder.folderPath
+                                path: ephemeralFolder.folder_path
                               }
                             })}
                           </div>