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/audio/voice-samples/+server.ts

@@ -0,0 +1,33 @@
+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';
+
+/**
+ * POST /api/audio/voice-samples
+ * Triggers generation of voice sample audio files for all (or specified) voices.
+ * Proxies to the scraper's POST /api/audio/voice-samples endpoint.
+ * Optional body: { voices: string[] } to generate a subset.
+ * Returns: { generated: string[], skipped: string[], failed: string[] }
+ */
+export const POST: RequestHandler = async ({ request }) => {
+	let body: { voices?: string[] } = {};
+	try {
+		body = await request.json();
+	} catch {
+		// Empty body is fine — generates all voices
+	}
+
+	try {
+		const res = await fetch(`${SCRAPER_URL}/api/audio/voice-samples`, {
+			method: 'POST',
+			headers: { 'Content-Type': 'application/json' },
+			body: JSON.stringify(body)
+		});
+		const data = await res.json();
+		return json(data, { status: res.ok ? 200 : res.status });
+	} catch (e) {
+		return json({ error: String(e) }, { status: 502 });
+	}
+};

ui/src/routes/api/presign/voice-sample/+server.ts

@@ -0,0 +1,26 @@
+import { json, error } from '@sveltejs/kit';
+import type { RequestHandler } from './$types';
+import { presignVoiceSample } from '$lib/server/minio';
+
+/**
+ * GET /api/presign/voice-sample?voice=af_bella
+ * Returns a presigned URL for the voice sample audio file.
+ * Returns 404 if the sample has not been generated yet.
+ */
+export const GET: RequestHandler = async ({ url }) => {
+	const voice = url.searchParams.get('voice');
+	if (!voice) {
+		error(400, 'Missing voice parameter');
+	}
+
+	try {
+		const presignedUrl = await presignVoiceSample(voice);
+		return json({ url: presignedUrl });
+	} catch (e) {
+		const status = (e as { status?: number }).status;
+		if (status === 404) {
+			error(404, 'Voice sample not found');
+		}
+		error(502, `Failed to presign voice sample: ${e}`);
+	}
+};

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: [] });
+	}
+};