Background-audio handoff for video + repository/player refactor

Hand video playback off to a native audio-only stream when the app is
backgrounded or locked, with no on-device video decode (UR-040). Adds
player_enter/exit_background_audio commands, an audio-only stream URL
for video items across the repository layer, and the frontend handoff
state machine wired into VideoPlayer. Includes accompanying
repository/offline/player refactoring and regenerates the traceability
matrix.

src/lib/utils/pictureInPicture.ts

@@ -1,6 +1,8 @@
 /**
  * Picture-in-picture support, Android only.
  *
+ * TRACES: UR-041 | IR-026 | DR-053
+ *
  * Video on Android renders into a native ExoPlayer SurfaceView behind the
  * WebView, so PiP is driven by the Activity (which shrinks into a floating
  * window) rather than the HTML5 `requestPictureInPicture()` API. The bridge is
@@ -68,8 +70,12 @@ export function enterPip(): void {
 /**
  * Enable/disable auto-entering PiP when the user backgrounds the app.
  *
- * Disabled while casting: playback is happening on another device, so a PiP
- * window here would render an empty black box.
+ * This is a coarse frontend override; the authoritative gate is the native
+ * `canEnterPip` guard, which already refuses PiP unless a local video surface
+ * is actively rendering (so audio playback, menu/library browsing, and
+ * remote/cast sessions never enter PiP regardless of this flag). The only
+ * caller today is the background-audio toggle, which disarms auto-PiP so the
+ * two background behaviours stay mutually exclusive.
  */
 export function setAutoEnterEnabled(enabled: boolean): void {
   try {