fix(audio): don't reset prefetch state for the chapter being consumed

The stale-prefetch reset effect compared prefetchedFor against nextChapter
only, so when landing on chapter N+1 (prefetchedFor=N+1, nextChapter=N+2)
it would destroy the pre-fetched URL before startPlayback() could use it,
breaking every auto-next transition after the first.

Fix: also allow prefetchedFor === chapter (current page) so the URL
survives long enough to be consumed, then only wipe truly foreign values.

ui/src/lib/components/AudioPlayer.svelte

@@ -78,11 +78,16 @@
 	});
 
 	// Reset next-chapter prefetch state when this chapter changes (new page).
-	// We only reset if the prefetch is for a *different* chapter than nextChapter
-	// (i.e. stale data from a prior page).
+	// Only reset if the prefetch belongs to neither the current chapter
+	// (about to be consumed by startPlayback) nor the next chapter (still valid).
+	// Any other value means stale data from a previous page.
 	$effect(() => {
 		const prefetchedFor = audioStore.nextChapterPrefetched;
-		if (prefetchedFor !== null && prefetchedFor !== (nextChapter ?? null)) {
+		if (
+			prefetchedFor !== null &&
+			prefetchedFor !== chapter &&
+			prefetchedFor !== (nextChapter ?? null)
+		) {
 			audioStore.resetNextPrefetch();
 		}
 	});