feat(ui): persistent cross-navigation audio player with expanded controls

- Add audio.svelte.ts: module singleton AudioStore (Svelte 5 runes) with
  slug/chapter/title metadata, status, progress, playback state, and
  toggleRequest/seekRequest signals for layout<->audio element communication
- Rewrite AudioPlayer.svelte as a store controller: no <audio> element owned;
  drives audioStore for presign->generate->play flow; shows inline controls
  when current chapter is active, 'Now playing / Load this chapter' banner
  when a different chapter is playing
- Update +layout.svelte: single persistent <audio> outside {#key} block so
  it never unmounts on navigation; effects to load URL, sync speed, handle
  toggle/seek requests; fixed bottom mini-player bar with seek, skip 15s/30s,
  speed cycle, go-to-chapter link, dismiss; pb-24 padding when active
- Pass chapterTitle and bookTitle from chapter page to AudioPlayer

ui/src/routes/books/[slug]/chapters/[n]/+page.svelte

@@ -67,7 +67,12 @@
 </div>
 
 <!-- Audio player -->
-<AudioPlayer slug={data.book.slug} chapter={data.chapter.number} />
+<AudioPlayer
+	slug={data.book.slug}
+	chapter={data.chapter.number}
+	chapterTitle={data.chapter.title || `Chapter ${data.chapter.number}`}
+	bookTitle={data.book.title}
+/>
 
 <!-- Chapter content -->
 {#if !data.html}