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>

lib/ttsPresets.ts

@@ -0,0 +1,77 @@
+// Xiaomi MiMo TTS endpoint presets.
+//
+// Xiaomi issues two independent key types, each with its own base URL:
+//   - Token Plan (套餐, `tp-` key): per-region endpoints token-plan-{sgp,cn,ams}.
+//   - Pay-as-you-go (按量, `sk-` key): the single unified endpoint api.xiaomimimo.com.
+//
+// Used CLIENT-SIDE ONLY: when a user supplies their own key, the browser calls
+// one of these endpoints directly (all return permissive CORS allowing the
+// `api-key` header), so the key never transits our server. Every endpoint
+// serves the same `mimo-v2.5-tts` family; Token Plan users pick the region
+// matching their subscription (also the closest hop → lower synth latency),
+// pay-as-you-go users have no region to choose. See docs/xiaomi-tts-key.md.
+
+export type TtsPreset = {
+  id: string;
+  /** Which key family this endpoint serves — drives the two-step picker UI. */
+  kind: "token-plan" | "payg";
+  /** Human label shown in the picker (region for Token Plan, type for payg). */
+  label: string;
+  /** OpenAI-style base; the TTS adapter appends `/chat/completions`. */
+  baseUrl: string;
+};
+
+/** Base model name; the adapter derives `-voicedesign` / `-voiceclone`. */
+export const DEFAULT_TTS_SPEECH_MODEL = "mimo-v2.5-tts";
+
+/**
+ * In-repo tutorial for getting a free Xiaomi MiMo key + picking a region.
+ * Points at the default branch so it resolves once this lands on main (which
+ * is what production serves). Linked from the homepage BYO modal, the play
+ * page's silence nudge, and the README.
+ */
+export const TTS_KEY_DOC_URL =
+  "https://github.com/zonghaoyuan/infiplot/blob/main/docs/xiaomi-tts-key.md";
+
+export const TTS_PRESETS: TtsPreset[] = [
+  {
+    id: "sgp",
+    kind: "token-plan",
+    label: "新加坡 · Singapore",
+    baseUrl: "https://token-plan-sgp.xiaomimimo.com/v1",
+  },
+  {
+    id: "cn",
+    kind: "token-plan",
+    label: "中国大陆 · China",
+    baseUrl: "https://token-plan-cn.xiaomimimo.com/v1",
+  },
+  {
+    id: "ams",
+    kind: "token-plan",
+    label: "欧洲 · Amsterdam",
+    baseUrl: "https://token-plan-ams.xiaomimimo.com/v1",
+  },
+  {
+    id: "payg",
+    kind: "payg",
+    label: "按量付费 · Pay-as-you-go",
+    baseUrl: "https://api.xiaomimimo.com/v1",
+  },
+];
+
+/** Token Plan endpoints only — the region sub-options shown once the user
+ *  picks the "套餐" key type. */
+export const TTS_REGION_PRESETS = TTS_PRESETS.filter(
+  (p) => p.kind === "token-plan",
+);
+
+/** The single pay-as-you-go preset id (`sk-` keys have no region). */
+export const PAYG_PRESET_ID = "payg";
+
+export function findTtsPreset(
+  id: string | null | undefined,
+): TtsPreset | undefined {
+  if (!id) return undefined;
+  return TTS_PRESETS.find((p) => p.id === id);
+}