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/providers/root-providers.tsx

@@ -3,26 +3,35 @@
 /**
  * 根级 Client Providers 包装
  *
- * 本轮只做两件事：
- *   1. 透传 `children`，让 `app/layout.tsx` 保持 Server Component 的同时，
- *      在子树中按需渲染 Client-only 组件（如未来 `<AuthContextProvider>`）。
- *   2. 预置一个 `<div id="toast-portal" />` 挂载点，供后续 toast 系统
- *      通过 `createPortal(..., document.getElementById("toast-portal"))` 渲染。
+ * 把所有功能 Provider 串起来：
+ *   AuthProvider → UserProvider → SidebarProvider → ChatProvider
  *
- * 不引入任何运行时依赖；不挂载任何 Context。
+ * 它们始终挂载，保证各页面直接 `use*State()` / `use*Dispatch()` 即可，
+ * 无需在每个页面单独包裹。
  */
 
 import type { ReactNode } from "react";
 
+import { AuthProvider } from "@/contexts/auth/auth-context";
+import { ChatProvider } from "@/contexts/chat/chat-context";
+import { SidebarProvider } from "@/contexts/sidebar/sidebar-context";
+import { UserProvider } from "@/contexts/user/user-context";
+
 export interface RootProvidersProps {
   children: ReactNode;
 }
 
 export function RootProviders({ children }: RootProvidersProps) {
   return (
-    <>
-      {children}
-      <div id="toast-portal" />
-    </>
+    <AuthProvider>
+      <UserProvider>
+        <SidebarProvider>
+          <ChatProvider>
+            {children}
+            <div id="toast-portal" />
+          </ChatProvider>
+        </SidebarProvider>
+      </UserProvider>
+    </AuthProvider>
   );
 }