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.test.ts

@@ -138,6 +138,51 @@ describe("backgroundAudioHandoff", () => {
       });
       expect(plan.position).toBe(0);
     });
+
+    // An episode that ends while backgrounded advances in the backend. Reloading
+    // the video the player was mounted with brought back the PREVIOUS episode,
+    // at the new episode's timestamp.
+    //
+    // TRACES: UR-040, UR-023 | DR-296 | UT-265
+    it("switches to the item the backend advanced to", () => {
+      const plan = planHandoffReturn({
+        useHtml5Element: false,
+        position: 95,
+        wasPlaying: true,
+        nativeStateKind: "playing",
+        mountedItemId: "ep1",
+        resumeItemId: "ep2",
+      });
+      expect(plan.target).toBe("other-item");
+      expect(plan.itemId).toBe("ep2");
+      expect(plan.position).toBe(95);
+    });
+
+    it("reloads in place when the backend is still on the mounted item", () => {
+      const plan = planHandoffReturn({
+        useHtml5Element: true,
+        position: 95,
+        wasPlaying: true,
+        nativeStateKind: "playing",
+        mountedItemId: "ep1",
+        resumeItemId: "ep1",
+      });
+      expect(plan.target).toBe("html5-element");
+    });
+
+    it("reloads in place when the backend reports no item", () => {
+      // Queue emptied while backgrounded (e.g. the sleep timer): there is no
+      // other item to go to, so the mounted one is the best we have.
+      const plan = planHandoffReturn({
+        useHtml5Element: false,
+        position: 95,
+        wasPlaying: false,
+        nativeStateKind: undefined,
+        mountedItemId: "ep1",
+        resumeItemId: null,
+      });
+      expect(plan.target).toBe("native-backend");
+    });
   });
 });