feat(audio): add voice selector, voice samples, MediaSession cover art, and fix speed/voice bug

- Fix speed/voice bug: AudioPlayer no longer accepts speed/voice as props;
  startPlayback() reads audioStore.voice/speed directly instead of overwriting them
- Add GET /api/voices endpoint (Go) proxying Kokoro, cached in-memory
- Add POST /api/audio/voice-samples endpoint (Go) to pre-generate sample clips
  for all voices and store them in MinIO under _voice-samples/{voice}.mp3
- Add GET /api/presign/voice-sample/{voice} endpoint (Go)
- Add SvelteKit proxy routes: /api/voices, /api/presign/voice-sample, /api/audio/voice-samples
- Add presignVoiceSample() helper in minio.ts with proper host rewrite
- Pass book.cover through +page.server.ts -> +page.svelte -> AudioPlayer
- Set navigator.mediaSession.metadata on playback start so cover art,
  book title, and chapter title appear on phone lock screen / notification

ui/src/routes/api/voices/+server.ts

@@ -0,0 +1,23 @@
+import { json } from '@sveltejs/kit';
+import type { RequestHandler } from './$types';
+import { env } from '$env/dynamic/private';
+
+const SCRAPER_URL = env.SCRAPER_API_URL ?? 'http://localhost:8080';
+
+/**
+ * GET /api/voices
+ * Proxies the voice list from the scraper → Kokoro.
+ * Returns { voices: string[] }
+ */
+export const GET: RequestHandler = async () => {
+	try {
+		const res = await fetch(`${SCRAPER_URL}/api/voices`);
+		if (!res.ok) {
+			return json({ voices: [] });
+		}
+		const data = (await res.json()) as { voices: string[] };
+		return json({ voices: data.voices ?? [] });
+	} catch {
+		return json({ voices: [] });
+	}
+};