feat: custom JWT auth + i18n with Chinese default

Replace Clerk with self-contained JWT auth (bcryptjs + jose), remove all
external auth dependencies. Add full i18n system with Chinese as default
language and English toggle — React Context + JSON dictionaries, no
external i18n library.

Auth:
- Add passwordHash to User model, generate JWT secret
- Create /api/auth/register, login, logout, me endpoints
- Rewrite proxy.ts middleware for custom session validation
- Add AuthProvider + useAuth hook, sign-in/sign-up pages
- Wire auth into layout and all API routes

i18n:
- React Context I18nProvider with localStorage + cookie persistence
- dictionaries/zh.ts (default) and dictionaries/en.ts
- LanguageSwitcher component, server-side createServerT() utility
- CJK font stack via [lang="zh"] CSS selector
- HTML lang attribute synced on language change
- All pages, components, TabBar, StageBadge translated
- Engine fallback strings (reflection-analyzer, summary-generator)
- API routes pass locale from cookie to engines

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/hooks/use-auth.ts

@@ -0,0 +1,25 @@
+"use client";
+
+import { useEffect, useState } from "react";
+
+interface User {
+  email: string;
+}
+
+export function useAuth() {
+  const [user, setUser] = useState<User | null>(null);
+  const [isLoaded, setIsLoaded] = useState(false);
+
+  useEffect(() => {
+    fetch("/api/auth/me")
+      .then((r) => {
+        if (!r.ok) throw new Error("Unauthorized");
+        return r.json();
+      })
+      .then((json) => setUser({ email: json.email }))
+      .catch(() => setUser(null))
+      .finally(() => setIsLoaded(true));
+  }, []);
+
+  return { user, isLoaded };
+}