fix(audio): replace autoStartPending bool with autoStartChapter number

The boolean flag was set by onended before goto() resolved, causing the
still-mounted outgoing chapter's AudioPlayer $effect to fire and call
startPlayback() for the wrong (old) chapter — restarting it from scratch.

Replace with autoStartChapter (number | null): the AudioPlayer only acts
when its own chapter prop === autoStartChapter, so the outgoing component
never matches and the incoming one fires exactly once on mount.

ui/src/lib/components/AudioPlayer.svelte

@@ -70,9 +70,11 @@
 	});
 
 	// Auto-start: if the layout navigated here via auto-next, kick off playback.
+	// We match against the chapter prop so the outgoing chapter's AudioPlayer
+	// (still mounted during the brief navigation window) never reacts to this.
 	$effect(() => {
-		if (audioStore.autoStartPending) {
-			audioStore.autoStartPending = false;
+		if (audioStore.autoStartChapter === chapter) {
+			audioStore.autoStartChapter = null;
 			startPlayback();
 		}
 	});