feat(ui): migrate Flutter UI library to Next.js 16 (App Router)

Migrate all 87 Dart files from /Users/chase/Documents/cozsweet/lib/ui/
to TypeScript React components, replacing Flutter-native features with
Web equivalents and reimplementing the BLoC pattern with React
Context + useReducer.

Highlights
- 1:1 BLoC -> Context+useReducer+side-effects mapping for auth, chat,
  user, and sidebar features (4 providers wired in root-providers.tsx)
- 500px MobileShell primitive eliminates per-screen layout boilerplate
- Headless Dialog primitive (createPortal + ESC + scroll lock) powers
  quota / pwa-install / username / pronouns / subscription / other-options
  dialogs
- New integrations/: facebook-sdk, google-identity, pwa-install,
  media-recorder (Web Speech API + MediaRecorder), browser-detect,
  chat-websocket (singleton), message-queue (throttled send)
- CSS Modules + existing design tokens (no new styling system);
  extended with breakpoints.css and dimensions.css
- Custom hooks: useBreakpoint (useSyncExternalStore),
  useFullVisibility (IntersectionObserver), usePwaInstall,
  usePullToRefresh
- Vitest + jsdom; 15 unit tests (chat-reducer, auth-validators)
- React 19.2.4, Next.js 16.2.7, Zod 4, lottie-react, classnames

Workarounds
- next.config.ts: experimental.staticGenerationRetryCount = 0 to
  bypass Next.js 16.2.7 /_global-error workStore prerender bug
- src/app/global-error.tsx: required Client Component
- src/types/globals.d.ts: centralized window.google / FB /
  SpeechRecognition declarations

Routes (/, /splash, /auth, /chat, /chat/deviceid/[deviceId],
/sidebar) all build and lint clean.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

src/contexts/user/user-reducer.ts

@@ -0,0 +1,50 @@
+/**
+ * 纯 user reducer
+ */
+import { type UserEvent, type UserState } from "./user-types";
+
+export function userReducer(state: UserState, event: UserEvent): UserState {
+  switch (event.type) {
+    case "_SetLoading":
+      return { ...state, isLoading: event.isLoading };
+    case "_SetUser":
+      return { ...state, currentUser: event.user };
+    case "_SetAvatarUrl":
+      return { ...state, avatarUrl: event.url };
+    case "_SetPronouns":
+      return { ...state, pronouns: event.pronouns };
+    case "_SetCoinBalance":
+      return { ...state, coinBalance: event.coinBalance };
+
+    case "UserUpdate":
+      return { ...state, currentUser: event.user };
+
+    case "UserUpdateUsername": {
+      if (!state.currentUser) return state;
+      return {
+        ...state,
+        currentUser: { ...state.currentUser, username: event.username },
+      };
+    }
+
+    case "UserUpdatePronouns":
+      return { ...state, pronouns: event.pronouns };
+
+    case "UserLogout":
+      return {
+        ...state,
+        currentUser: null,
+        avatarUrl: null,
+      };
+
+    case "UserInit":
+    case "UserFetch":
+    case "UserDeleteChatHistory":
+    case "UserDeleteAccount":
+      // 由 side-effects 处理
+      return state;
+
+    default:
+      return state;
+  }
+}