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/app/globals.css

@@ -5,6 +5,8 @@
 @import "../design/radius.css";
 @import "../design/border-widths.css";
 @import "../design/icon-sizes.css";
+@import "../design/breakpoints.css";
+@import "../design/dimensions.css";
 
 /* ============================================================
  * Tailwind 4 @theme - 将 CSS 变量注册为 utility 类
@@ -59,6 +61,19 @@
   );
   --color-page-background: var(--color-page-background);
 
+  /* 扩展色 */
+  --color-sidebar-background: var(--color-sidebar-background);
+  --color-dark-background: var(--color-dark-background);
+  --color-dark-gray: var(--color-dark-gray);
+  --color-bubble-background: var(--color-bubble-background);
+  --color-chat-input-border: var(--color-chat-input-border);
+  --color-input-placeholder: var(--color-input-placeholder);
+  --color-pwa-button-text: var(--color-pwa-button-text);
+  --color-facebook-blue: var(--color-facebook-blue);
+  --color-dialog-background: var(--color-dialog-background);
+  --color-dialog-border: var(--color-dialog-border);
+  --color-blur-background: var(--color-blur-background);
+
   /* ==================== 间距 ==================== */
   --spacing-xs: var(--spacing-xs);
   --spacing-sm: var(--spacing-sm);
@@ -78,7 +93,7 @@
 
   /* ==================== 字体 ==================== */
   --font-sans: var(--font-system);
-  --font-athelas: var(--font-athelas);
+  --font-athelas: var(--color-athelas, var(--font-athelas));
 
   /* ==================== 边框宽度 ==================== */
   --border-thin: var(--border-thin);