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/lib/audio.svelte.ts

@@ -0,0 +1,60 @@
+/**
+ * Global audio player state for libnovel.
+ *
+ * A single shared instance (module singleton) keeps audio playing across
+ * SvelteKit navigations.  The layout mounts the <audio> element once and
+ * never unmounts it; the per-chapter AudioPlayer component is just a
+ * controller that reads/writes this state.
+ *
+ * Uses Svelte 5 runes ($state / $derived) — import only from .svelte files
+ * or other .svelte.ts files.
+ */
+
+export type AudioStatus = 'idle' | 'loading' | 'generating' | 'ready' | 'error';
+
+class AudioStore {
+	// ── What is loaded ──────────────────────────────────────────────────────
+	slug = $state('');
+	chapter = $state(0);
+	chapterTitle = $state('');
+	bookTitle = $state('');
+	voice = $state('af_bella');
+	speed = $state(1.0);
+
+	// ── Loading/generation state ────────────────────────────────────────────
+	status = $state<AudioStatus>('idle');
+	audioUrl = $state('');
+	errorMsg = $state('');
+	/** Pseudo-progress bar value 0–100 during generation */
+	progress = $state(0);
+
+	// ── Playback state (kept in sync with the <audio> element) ─────────────
+	currentTime = $state(0);
+	duration = $state(0);
+	isPlaying = $state(false);
+
+	/**
+	 * Increment to signal the layout to toggle play/pause.
+	 * The layout watches this with $effect and calls audioEl.play()/pause().
+	 */
+	toggleRequest = $state(0);
+
+	/**
+	 * Set to a number to seek the audio element to that time (seconds).
+	 * The layout watches this with $effect and sets audioEl.currentTime.
+	 * Reset to null after handling.
+	 */
+	seekRequest = $state<number | null>(null);
+
+	/** Whether the mini-bar at the bottom is visible */
+	get active(): boolean {
+		return this.status === 'ready' || this.status === 'generating' || this.status === 'loading';
+	}
+
+	/** True when the currently loaded track matches slug+chapter */
+	isCurrentChapter(slug: string, chapter: number): boolean {
+		return this.slug === slug && this.chapter === chapter;
+	}
+}
+
+export const audioStore = new AudioStore();