fix: announce chapter reliability — timeout fallback + eager chapters sync

Two issues causing announce to silently fail or permanently block navigation:

1. No hard timeout fallback on speechSynthesis.speak():
   Chrome Android (and some desktop) silently drops utterances not triggered
   within a user-gesture window. If both onend and onerror fail to fire (a
   known browser bug), doNavigate() was never called and the chapter
   transition was permanently lost. Added an 8-second setTimeout fallback
   (safeNavigate) that forces navigation if the speech engine never resolves.
   safeNavigate is idempotent — guarded by a 'navigated' flag so it only
   fires once even if onend, onerror, and the timeout all fire.

2. audioStore.chapters only written inside startPlayback():
   The onended handler reads audioStore.chapters to build the utterance text
   (Chapter N — Title). If auto-next navigated to this chapter and the user
   never manually pressed play (startPlayback was never called), chapters
   held whatever the previous AudioPlayer had written — potentially stale or
   empty on a book switch. Added a reactive $effect that keeps chapters in
   sync whenever the prop changes, same pattern as nextChapter.

ui/src/lib/components/AudioPlayer.svelte

@@ -248,6 +248,12 @@
 		audioStore.nextChapter = nextChapter ?? null;
 	});
 
+	// Keep chapters list in store up to date so the layout's onended announce
+	// can find titles even if startPlayback() hasn't been called yet on this mount.
+	$effect(() => {
+		if (chapters.length > 0) audioStore.chapters = chapters;
+	});
+
 	// Keep voices in store up to date whenever prop changes.
 	$effect(() => {
 		if (voices.length > 0) audioStore.voices = voices;

ui/src/routes/+layout.svelte

@@ -406,8 +406,25 @@
 				const text = `Chapter ${targetChapter}${titlePart}`;
 				window.speechSynthesis.cancel();
 				const utterance = new SpeechSynthesisUtterance(text);
-				utterance.onend = doNavigate;
+
-				utterance.onerror = doNavigate;
+				// Guard: ensure doNavigate can only fire once even if both
+				// onend and the timeout fire, or onerror fires after onend.
+				let navigated = false;
+				const safeNavigate = () => {
+					if (navigated) return;
+					navigated = true;
+					clearTimeout(announceTimeout);
+					doNavigate();
+				};
+
+				// Hard fallback: if speechSynthesis silently drops the utterance
+				// (common on Chrome Android due to gesture policy, or when the
+				// browser is busy fetching the next chapter's audio), navigate
+				// anyway after a generous 8-second window.
+				const announceTimeout = setTimeout(safeNavigate, 8000);
+
+				utterance.onend = safeNavigate;
+				utterance.onerror = safeNavigate;
 				window.speechSynthesis.speak(utterance);
 			} else {
 				doNavigate();