fix: auto-next transition deadlock and resume-at-end bug

Bug 1 — Auto-next not transitioning:
audioExpanded defaulted to false on the new chapter page because
audioStore.chapter still held the old chapter number when the page script
initialized. The $effect only opened the panel when isPlaying was already
true — a circular dependency (can't play without the panel, panel only opens
when playing). Fix: also set audioExpanded=true when autoStartChapter targets
this chapter, both in the initial $state and in the reactive $effect.

Bug 2 — Resume starts at the end:
onended called saveAudioTime() which captured currentTime≈duration and fired a
PATCH 2 seconds later (after navigation had already completed). Next visit to
that chapter restored the end-of-file position. Fix: in onended, cancel the
debounced timer (clearTimeout) and immediately PATCH audioTime=0 for the
finished chapter, so it always resumes from the beginning on re-visit.

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;

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;
 		}
 	});