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/routes/+layout.svelte

@@ -184,9 +184,11 @@
 			// we need.
 			const targetSlug = audioStore.slug;
 			const targetChapter = audioStore.nextChapter;
-			audioStore.autoStartPending = true;
+			// Store the target chapter number so only the newly-mounted AudioPlayer
+			// for that chapter reacts — not the outgoing chapter's component.
+			audioStore.autoStartChapter = targetChapter;
 			goto(`/books/${targetSlug}/chapters/${targetChapter}`).catch(() => {
-				audioStore.autoStartPending = false;
+				audioStore.autoStartChapter = null;
 			});
 		}
 	}}