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>

README.en.md

@@ -159,6 +159,12 @@ With the recommended trio, each scene's cost comes mainly from the image generat
 
 By default the browser fetches images directly from the provider — no setup needed; leave `NEXT_PUBLIC_IMAGE_PROXY_URL` blank and you're completely unaffected. You only want this if you hit progressive "top-to-bottom" image loading (Chrome's `ERR_QUIC_PROTOCOL_ERROR` on some networks paints partial PNGs row by row): deploy a tiny Cloudflare Worker that re-fetches images server-side and serves them atomically over HTTP/2. One-click deploy at **[infiplot-image-proxy](https://github.com/zonghaoyuan/infiplot-image-proxy)**, then paste the `workers.dev` URL it prints into `NEXT_PUBLIC_IMAGE_PROXY_URL`.
 
+**5. Let players bring their own voice Key (optional, recommended)**
+
+Xiaomi rate-limits the TTS model by RPM/TPM. When a public deployment has many people playing at once through a single shared `TTS_API_KEY`, those limits are easy to hit — the symptom is **story and visuals work fine, but there's no audio**. To fix this, players can optionally enter **their own** Xiaomi MiMo key on the homepage (free to obtain). Synthesis then runs **browser-direct to Xiaomi**, the **key stays in the player's browser and never touches your server**, and they get stable voice with lower latency. It's purely additive: leave it blank and playback falls back to your server key exactly as before.
+
+See the [Bring-your-own voice Key guide](docs/xiaomi-tts-key.md) for how to obtain and enter one.
+
 ---
 
 ## Roadmap