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>

prisma/schema.prisma

@@ -4,7 +4,7 @@ generator client {
 }
 
 datasource db {
-  provider = "postgresql"
+  provider = "sqlite"
 }
 
 enum TaskType {
@@ -32,9 +32,10 @@ enum RewardEventType {
 }
 
 model User {
-  id         String    @id // Clerk user ID
-  email      String    @unique
-  createdAt  DateTime  @default(now()) @map("created_at")
+  id           String    @id @default(uuid())
+  email        String    @unique
+  passwordHash String?   @map("password_hash")
+  createdAt    DateTime  @default(now()) @map("created_at")
 
   state         UserState?     @relation("UserState")
   sessions      Session[]