feat(web): optional bring-your-own Xiaomi MiMo TTS key (browser-side synthesis)

Public users share one server TTS key, so Xiaomi's per-key RPM/TPM limits
cause silent playback under concurrency. This adds an OPTIONAL path: a user
can store their own Xiaomi MiMo key in the browser and synthesize voice
client-side against Xiaomi's CORS-open endpoints. The key lives only in
localStorage and is never sent to or logged by our server; the shared server
key still serves everyone who does not opt in.

- components/TtsKeyModal.tsx: shared key modal (key-family + region picker),
  reused by both the home and play pages
- app/play/page.tsx: silence nudge moved beside the mute toggle; modal opens
  in place instead of redirecting to the home page
- app/page.tsx: home page consumes the shared modal + readStoredTtsConfig
- lib/clientTtsConfig.ts, lib/ttsPresets.ts: browser config + region presets
- app/api/{start,scene,insert-beat}: thread per-request voice; lib/types update
- docs/xiaomi-tts-key.md + README note

Verified with tsc --noEmit (exit 0).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

app/api/start/route.ts

@@ -41,7 +41,11 @@ export async function POST(req: Request) {
   }
 
   try {
-    const config = loadEngineConfig(req.headers);
+    const base = loadEngineConfig(req.headers);
+    // BYO key: the browser provisions + synths voices directly against Xiaomi
+    // (key never reaches us), so strip server-side TTS so the engine skips all
+    // provisioning + synth. See StartRequest.clientTts.
+    const config = body.clientTts ? { ...base, tts: undefined } : base;
     const result = await startSession(config, body);
     return NextResponse.json(result);
   } catch (err) {