feat: add MPRIS media controls integration

Backend (Rust):
- Add souvlaki crate for MPRIS support on Linux
- Create media_controls module with MediaControlsManager
- Track metadata (title, artist, album, duration, cover)
- Playback state (playing/paused/stopped)
- Wire pause/resume/stop commands to update MPRIS
- Add set_media_metadata Tauri command

Frontend (Svelte):
- Call set_media_metadata when track starts playing
- Send track info to MPRIS for system integration

Now shows track info in system media controls and
supports media key events (logged for now).

src-tauri/src/commands/playback.rs

@@ -24,6 +24,7 @@ pub async fn play_track(track_id: u64, state: State<'_, AppState>) -> Result<(),
 #[tauri::command]
 pub fn pause_playback(state: State<'_, AppState>) -> Result<(), String> {
     log::info!("Command: pause_playback");
+    state.media_controls.set_playback(false);
     state.player.pause()
 }
 
@@ -31,6 +32,7 @@ pub fn pause_playback(state: State<'_, AppState>) -> Result<(), String> {
 #[tauri::command]
 pub fn resume_playback(state: State<'_, AppState>) -> Result<(), String> {
     log::info!("Command: resume_playback");
+    state.media_controls.set_playback(true);
     state.player.resume()
 }
 
@@ -38,9 +40,33 @@ pub fn resume_playback(state: State<'_, AppState>) -> Result<(), String> {
 #[tauri::command]
 pub fn stop_playback(state: State<'_, AppState>) -> Result<(), String> {
     log::info!("Command: stop_playback");
+    state.media_controls.set_stopped();
     state.player.stop()
 }
 
+/// Set media controls metadata (for MPRIS integration)
+#[tauri::command]
+pub fn set_media_metadata(
+    title: String,
+    artist: String,
+    album: String,
+    duration_secs: Option<u64>,
+    cover_url: Option<String>,
+    state: State<'_, AppState>,
+) -> Result<(), String> {
+    log::info!("Command: set_media_metadata - {} by {}", title, artist);
+    crate::update_media_controls_metadata(
+        &state.media_controls,
+        &title,
+        &artist,
+        &album,
+        duration_secs,
+        cover_url,
+    );
+    state.media_controls.set_playback(true);
+    Ok(())
+}
+
 /// Set volume (0.0 - 1.0)
 #[tauri::command]
 pub fn set_volume(volume: f32, state: State<'_, AppState>) -> Result<(), String> {