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/routes/player/[id]/+page.svelte

@@ -50,6 +50,9 @@
   // When advancing to a next episode we always start from the beginning,
   // even if the episode was previously started or watched.
   const restartParam = $derived($page.url.searchParams.get("restart") === "true");
+  // Explicit start position in seconds — set when returning from background
+  // audio onto an episode the backend advanced to (DR-296).
+  const resumeAtParam = $derived(Number($page.url.searchParams.get("resumeAt")) || 0);
 
   // Derive playback context from URL query params
   const playbackContext = $derived.by(() => {
@@ -121,6 +124,7 @@
   $effect(() => {
     const id = itemId;
     const restart = restartParam;
+    const resumeAt = resumeAtParam;
     if (id && id !== loadedItemId) {
       autoPlayLog.debug(
         "$effect triggered: loading new item",
@@ -132,7 +136,11 @@
       );
       // restart=true (advancing to next episode) forces start-from-beginning,
       // bypassing the resume-progress check.
-      loadAndPlay(id, restart ? 0 : undefined, restart);
+      if (resumeAt > 0) {
+        loadAndPlay(id, resumeAt);
+      } else {
+        loadAndPlay(id, restart ? 0 : undefined, restart);
+      }
     }
   });
 
@@ -797,6 +805,23 @@
     }
   }
 
+  /**
+   * Background audio advanced to another episode; show that one where the audio
+   * left off, rather than the episode this page was opened on.
+   *
+   * The outgoing episode was played out (the backend advances only past its
+   * end), so it is recorded as watched and the VideoPlayer's unmount stop
+   * report — which would carry the stale handoff position — is suppressed, as
+   * for a manual skip.
+   *
+   * TRACES: UR-040, UR-023 | DR-296
+   */
+  function handleResumeOtherItem(nextId: string, positionSeconds: number) {
+    void reportSkippedEpisode(currentMedia?.id ?? itemId ?? null);
+    const start = positionSeconds > 0 ? `resumeAt=${Math.floor(positionSeconds)}` : "restart=true";
+    goto(`/player/${nextId}?${start}`, { replaceState: true });
+  }
+
   function handleSkipToNextEpisode() {
     if (nextEpisode) {
       // Skipping means "I'm done with this one" — record the outgoing episode as
@@ -889,6 +914,7 @@
     onEnded={handleVideoEnded}
     hasNext={nextEpisode !== null}
     onNext={handleSkipToNextEpisode}
+    onResumeOtherItem={handleResumeOtherItem}
   />
   <NextEpisodePopup />
 {:else}