fix(audio): fix auto-next reliability issues

- Clear autoStartPending if goto() fails to avoid spurious auto-starts on future navigations
- Clear audioStore.nextChapter on AudioPlayer unmount so a stale chapter can't trigger navigation after leaving a chapter page
- Remove redundant nextChapter write in startPlayback() — the  already keeps it in sync

ui/src/lib/components/AudioPlayer.svelte

@@ -46,8 +46,13 @@
 
 	// Keep nextChapter in the store so the layout's onended can navigate.
 	// Run as an effect so it stays in sync if the prop ever changes.
+	// On unmount, clear it so a stale value can't trigger navigation after
+	// the user leaves the chapter page.
 	$effect(() => {
 		audioStore.nextChapter = nextChapter ?? null;
+		return () => {
+			audioStore.nextChapter = null;
+		};
 	});
 
 	// Auto-start: if the layout navigated here via auto-next, kick off playback.
@@ -128,7 +133,7 @@
 		audioStore.bookTitle = bookTitle;
 		audioStore.voice = voice;
 		audioStore.speed = speed;
-		audioStore.nextChapter = nextChapter ?? null;
+		// nextChapter is kept in sync by the $effect above — no need to write it here
 		audioStore.status = 'loading';
 		audioStore.errorMsg = '';
 

ui/src/routes/+layout.svelte

@@ -179,9 +179,11 @@
 		audioStore.isPlaying = false;
 		saveAudioTime();
 		if (audioStore.autoNext && audioStore.nextChapter !== null && audioStore.slug) {
-			audioStore.autoStartPending = true;
-			goto(`/books/${audioStore.slug}/chapters/${audioStore.nextChapter}`);
-		}
+				audioStore.autoStartPending = true;
+				goto(`/books/${audioStore.slug}/chapters/${audioStore.nextChapter}`).catch(() => {
+					audioStore.autoStartPending = false;
+				});
+			}
 	}}
 	preload="metadata"
 	style="display:none"