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/hooks/use-pull-to-refresh.ts

@@ -0,0 +1,69 @@
+"use client";
+/**
+ * 下拉刷新 hook（占位实现）
+ *
+ * 原始 Dart: `easy_refresh` 包的 `EasyRefresh.builder`（chat_area.dart 用来加载更多历史消息）。
+ *
+ * 当前实现：最小可用版本 —— 监听顶部 overscroll 触发 onRefresh。
+ * 后续可替换为 react-use / react-easy-refresh。
+ */
+import { useEffect, useRef, useState } from "react";
+
+export function usePullToRefresh(
+  onRefresh: () => Promise<void> | void,
+  options: { threshold?: number; disabled?: boolean } = {},
+): {
+  isRefreshing: boolean;
+  pullDistance: number;
+  containerRef: React.RefObject<HTMLDivElement | null>;
+} {
+  const { threshold = 80, disabled = false } = options;
+  const [isRefreshing, setIsRefreshing] = useState(false);
+  const [pullDistance, setPullDistance] = useState(0);
+  const startY = useRef<number | null>(null);
+  const containerRef = useRef<HTMLDivElement | null>(null);
+
+  useEffect(() => {
+    if (disabled || typeof window === "undefined") return;
+    const el = containerRef.current;
+    if (!el) return;
+
+    const onTouchStart = (e: TouchEvent) => {
+      if (el.scrollTop > 0) return;
+      startY.current = e.touches[0]?.clientY ?? null;
+    };
+    const onTouchMove = (e: TouchEvent) => {
+      if (startY.current == null) return;
+      const y = e.touches[0]?.clientY ?? startY.current;
+      const delta = y - startY.current;
+      if (delta > 0 && el.scrollTop <= 0) {
+        setPullDistance(Math.min(delta, threshold * 1.5));
+      }
+    };
+    const onTouchEnd = async () => {
+      if (startY.current == null) return;
+      if (pullDistance >= threshold && !isRefreshing) {
+        setIsRefreshing(true);
+        try {
+          await onRefresh();
+        } finally {
+          setIsRefreshing(false);
+          setPullDistance(0);
+        }
+      } else {
+        setPullDistance(0);
+      }
+      startY.current = null;
+    };
+    el.addEventListener("touchstart", onTouchStart, { passive: true });
+    el.addEventListener("touchmove", onTouchMove, { passive: true });
+    el.addEventListener("touchend", onTouchEnd);
+    return () => {
+      el.removeEventListener("touchstart", onTouchStart);
+      el.removeEventListener("touchmove", onTouchMove);
+      el.removeEventListener("touchend", onTouchEnd);
+    };
+  }, [threshold, disabled, isRefreshing, onRefresh, pullDistance]);
+
+  return { isRefreshing, pullDistance, containerRef };
+}