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/app/globals.css

@@ -44,3 +44,13 @@ body {
   -webkit-font-smoothing: antialiased;
   -moz-osx-font-smoothing: grayscale;
 }
+
+/* CJK font stack — wider characters read better at slightly smaller sizes */
+[lang="zh"] {
+  --font-display: "Noto Serif SC", "Source Han Serif SC", var(--font-fraunces), Georgia, serif;
+  --font-body: "Noto Sans SC", "Source Han Sans SC", "PingFang SC", "Microsoft YaHei", var(--font-dm-sans), system-ui, sans-serif;
+}
+
+[lang="zh"] body {
+  font-size: 15px;
+}