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/player/backgroundAudio.commands.test.ts

@@ -32,12 +32,12 @@ describe("background-audio player commands (param naming)", () => {
     });
   });
 
-  it("player_exit_background_audio takes no params and returns a position", async () => {
-    (invoke as any).mockResolvedValueOnce(193.5);
+  it("player_exit_background_audio takes no params and returns the resume point", async () => {
+    (invoke as any).mockResolvedValueOnce({ itemId: "ep2", positionSeconds: 193.5 });
 
-    const pos = await commands.playerExitBackgroundAudio();
+    const resume = await commands.playerExitBackgroundAudio();
 
-    expect(pos).toBe(193.5);
+    expect(resume).toEqual({ itemId: "ep2", positionSeconds: 193.5 });
     expect(invoke).toHaveBeenCalledWith("player_exit_background_audio");
   });
 });