fix(player): return from background audio onto the episode it advanced to

An episode that ends while backgrounded in audio-only mode advances in the
backend, but player_exit_background_audio returned only a position, so the
video page reloaded the episode it was mounted with -- the previous one, at
the new episode's timestamp.

The command now returns BackgroundAudioResume { itemId, positionSeconds }.
planHandoffReturn yields "other-item" when the id differs from the mounted
one, and the player page navigates to that episode with resumeAt=<seconds>,
marking the outgoing episode watched and suppressing its stale stop report.

TRACES: UR-040, UR-023 | DR-296 | UT-265, UT-266

src/lib/components/player/backgroundAudioHandoff.ts

@@ -77,8 +77,14 @@ export function shouldResumeOnForeground(
 
 /** What has to be restarted to put picture back on screen, and how. */
 export interface HandoffReturn {
-  /** Which renderer must be brought back. */
-  target: "html5-element" | "native-backend";
+  /**
+   * Which renderer must be brought back — or `other-item` when the backend is
+   * no longer on the item this player was mounted with, so the player must
+   * switch to `itemId` instead of reloading itself.
+   */
+  target: "html5-element" | "native-backend" | "other-item";
+  /** The item to switch to; set only for `other-item`. */
+  itemId?: string;
   /** Absolute position the background audio reached. */
   position: number;
   /** Whether playback should be running once it is back. */
@@ -107,18 +113,31 @@ export interface HandoffReturn {
  * `shouldPlay` folds in [shouldResumeOnForeground], so a lockscreen pause during
  * the handoff still wins over the snapshot taken on the way out.
  *
- * TRACES: UR-040, UR-003 | DR-196 | UT-060
+ * An episode that ends while backgrounded advances in the backend, so the item
+ * the native player returns on (`resumeItemId`) can differ from the one this
+ * player was mounted with. Reloading the mounted one brought back the previous
+ * episode at the new one's timestamp; in that case the plan is `other-item`.
+ * A missing `resumeItemId` (queue emptied) keeps the in-place reload.
+ *
+ * TRACES: UR-040, UR-003, UR-023 | DR-196, DR-296 | UT-060, UT-265
  */
 export function planHandoffReturn(opts: {
   useHtml5Element: boolean;
   position: number;
   wasPlaying: boolean;
   nativeStateKind: string | undefined;
+  mountedItemId?: string | null;
+  resumeItemId?: string | null;
 }): HandoffReturn {
+  const position = opts.position > 0 ? opts.position : 0;
+  const shouldPlay = shouldResumeOnForeground(opts.wasPlaying, opts.nativeStateKind);
+  if (opts.resumeItemId && opts.resumeItemId !== opts.mountedItemId) {
+    return { target: "other-item", itemId: opts.resumeItemId, position, shouldPlay };
+  }
   return {
     target: opts.useHtml5Element ? "html5-element" : "native-backend",
-    position: opts.position > 0 ? opts.position : 0,
-    shouldPlay: shouldResumeOnForeground(opts.wasPlaying, opts.nativeStateKind),
+    position,
+    shouldPlay,
   };
 }