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

@@ -363,7 +363,20 @@
 	}}
 	onended={() => {
 		audioStore.isPlaying = false;
-		saveAudioTime();
+		// Cancel any pending debounced save and reset the position to 0 for
+		// the chapter that just finished. Without this, the 2s debounce fires
+		// after navigation and saves currentTime≈duration, causing resume to
+		// start at the very end next time the user returns to this chapter.
+		clearTimeout(audioTimeSaveTimer);
+		if (audioStore.slug && audioStore.chapter) {
+			const slug = audioStore.slug;
+			const chapter = audioStore.chapter;
+			fetch('/api/progress/audio-time', {
+				method: 'PATCH',
+				headers: { 'Content-Type': 'application/json' },
+				body: JSON.stringify({ slug, chapter, audioTime: 0 })
+			}).catch(() => {});
+		}
 		// If sleep-after-chapter is set, just pause instead of navigating
 		if (audioStore.sleepAfterChapter) {
 			audioStore.sleepAfterChapter = false;
@@ -393,8 +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();

ui/src/routes/books/[slug]/chapters/[n]/+page.svelte

@@ -311,14 +311,20 @@
 		return t || `Chapter ${data.chapter.number}`;
 	});
 
-	// Audio panel: auto-open if this chapter is already loaded/playing in the store
+	// Audio panel: auto-open if this chapter is already loaded/playing in the store,
+	// OR if auto-next is about to start it (autoStartChapter is set before navigation).
 	// svelte-ignore state_referenced_locally
 	let audioExpanded = $state(
-		audioStore.slug === data.book.slug && audioStore.chapter === data.chapter.number
+		(audioStore.slug === data.book.slug && audioStore.chapter === data.chapter.number) ||
+		audioStore.autoStartChapter === data.chapter.number
 	);
 	$effect(() => {
-		// Expand automatically when the store starts playing this chapter
-		if (audioStore.slug === data.book.slug && audioStore.chapter === data.chapter.number && audioStore.isPlaying) {
+		// Expand automatically when the store starts playing this chapter,
+		// or when auto-next targets this chapter (before startPlayback has run).
+		if (
+			(audioStore.slug === data.book.slug && audioStore.chapter === data.chapter.number && audioStore.isPlaying) ||
+			audioStore.autoStartChapter === data.chapter.number
+		) {
 			audioExpanded = true;
 		}
 	});