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/lib/server/minio.ts

@@ -61,6 +61,35 @@ export async function presignChapter(slug: string, n: number, rewrite = false):
 	return rewrite ? rewriteHost(data.url) : data.url;
 }
 
+/**
+ * Returns a presigned URL for a voice sample audio file.
+ * URL is valid for ~1 hour. The URL is returned to the browser for direct streaming.
+ * Throws with { status: 404 } when the sample has not been generated yet.
+ */
+export async function presignVoiceSample(voice: string): Promise<string> {
+	log.debug('minio', 'presigning voice sample', { voice });
+	let res: Response;
+	try {
+		res = await fetch(`${SCRAPER_URL}/api/presign/voice-sample/${encodeURIComponent(voice)}`);
+	} catch (e) {
+		log.error('minio', 'presign voice sample network error', { voice, err: String(e) });
+		throw new Error(`presign voice sample ${voice}: network error`);
+	}
+	if (res.status === 404) {
+		const err = new Error(`presign voice sample ${voice}: not found`) as Error & { status: number };
+		err.status = 404;
+		throw err;
+	}
+	if (!res.ok) {
+		const body = await res.text().catch(() => '');
+		log.error('minio', 'presign voice sample failed', { voice, status: res.status, body });
+		throw new Error(`presign voice sample ${voice}: ${res.status}`);
+	}
+	const data = (await res.json()) as { url: string };
+	log.debug('minio', 'presign voice sample ok', { voice });
+	return rewriteHost(data.url);
+}
+
 /**
  * Returns a presigned URL for an audio file.
  * URL is valid for ~1 hour. The URL is returned to the browser for direct streaming.