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/utils/app-env.ts

@@ -0,0 +1,15 @@
+/**
+ * 应用环境检测
+ *
+ * 原始 Dart: lib/utils/app_env_util.dart
+ *
+ * 替代 `AppEnvUtil.isDevelopment`：基于 Next.js 注入的 `process.env.NODE_ENV`。
+ */
+export const isDevelopment = (): boolean =>
+  process.env.NODE_ENV !== "production";
+
+export const isProduction = (): boolean =>
+  process.env.NODE_ENV === "production";
+
+export const isTest = (): boolean =>
+  process.env.NODE_ENV === "test";

src/utils/date.ts

@@ -0,0 +1,40 @@
+/**
+ * 日期工具
+ *
+ * 原始 Dart: lib/utils/date_utils.dart（app_date_utils.DateUtils）
+ *
+ * 业务侧用 `formatDate(date)` 生成聊天时间戳，`todayString()` 用于每日配额 key。
+ */
+
+/** 格式：`HH:mm`（与 Dart 端 `formatDate` 一致） */
+export function formatDate(date: Date = new Date()): string {
+  const hh = String(date.getHours()).padStart(2, "0");
+  const mm = String(date.getMinutes()).padStart(2, "0");
+  return `${hh}:${mm}`;
+}
+
+/** 格式：`YYYY-MM-DD`，用于每日配额 key。 */
+export function todayString(date: Date = new Date()): string {
+  const y = date.getFullYear();
+  const m = String(date.getMonth() + 1).padStart(2, "0");
+  const d = String(date.getDate()).padStart(2, "0");
+  return `${y}-${m}-${d}`;
+}
+
+/** 格式：人类可读的日期（`MMM D, YYYY`）。 */
+export function formatHumanDate(date: Date = new Date()): string {
+  return date.toLocaleDateString("en-US", {
+    month: "short",
+    day: "numeric",
+    year: "numeric",
+  });
+}
+
+/** 同一日期（忽略时分秒） */
+export function isSameDay(a: Date, b: Date): boolean {
+  return (
+    a.getFullYear() === b.getFullYear() &&
+    a.getMonth() === b.getMonth() &&
+    a.getDate() === b.getDate()
+  );
+}