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/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"