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>

package.json

@@ -9,9 +9,13 @@
     "lint": "eslint"
   },
   "dependencies": {
-    "@clerk/nextjs": "^7.3.7",
+
+    "@prisma/adapter-better-sqlite3": "^7.8.0",
     "@prisma/adapter-pg": "^7.8.0",
     "@prisma/client": "^7.8.0",
+    "bcryptjs": "^3.0.3",
+    "better-sqlite3": "^12.10.0",
+    "jose": "^6.2.3",
     "next": "16.2.6",
     "openai": "^6.38.0",
     "react": "19.2.4",
@@ -19,6 +23,7 @@
   },
   "devDependencies": {
     "@tailwindcss/postcss": "^4",
+    "@types/bcryptjs": "^3.0.0",
     "@types/node": "^20.19.41",
     "@types/react": "^19",
     "@types/react-dom": "^19",