feat(tts): dual-engine voice list (kokoro + pocket-tts)

Expose all available voices from both TTS engines via the /api/voices
endpoint. AudioPlayer and profile voice-selector now group voices by
engine and show a labelled optgroup. Voice type carries an engine field
so the chapter-reader can route synthesis to the correct backend.

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

@@ -1,11 +1,12 @@
 import { json } from '@sveltejs/kit';
 import type { RequestHandler } from './$types';
 import { backendFetch } from '$lib/server/scraper';
+import type { Voice } from '$lib/types';
 
 /**
  * GET /api/voices
- * Proxies the voice list from the backend → Kokoro.
- * Returns { voices: string[] }
+ * Proxies the voice list from the backend (Kokoro + pocket-tts).
+ * Returns { voices: Voice[] }
  */
 export const GET: RequestHandler = async () => {
 	try {
@@ -13,7 +14,7 @@ export const GET: RequestHandler = async () => {
 		if (!res.ok) {
 			return json({ voices: [] });
 		}
-		const data = (await res.json()) as { voices: string[] };
+		const data = (await res.json()) as { voices: Voice[] };
 		return json({ voices: data.voices ?? [] });
 	} catch {
 		return json({ voices: [] });