feat: add multi-language (i18n) support for vendor dashboard and storefront

- Add database fields for language preferences:
  - Vendor: dashboard_language, storefront_language, storefront_languages
  - User: preferred_language
  - Customer: preferred_language

- Add language middleware for request-level language detection:
  - Cookie-based persistence
  - Browser Accept-Language fallback
  - Vendor storefront language constraints

- Add language API endpoints (/api/v1/language/*):
  - POST /set - Set language preference
  - GET /current - Get current language info
  - GET /list - List available languages
  - DELETE /clear - Clear preference

- Add i18n utilities (app/utils/i18n.py):
  - JSON-based translation loading
  - Jinja2 template integration
  - Language resolution helpers

- Add reusable language selector macros for templates
- Add languageSelector() Alpine.js component
- Add translation files (en, fr, de, lb) in static/locales/
- Add architecture rules documentation for language implementation
- Update marketplace-product-detail.js to use native language names

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

models/database/user.py

@@ -46,6 +46,10 @@ class User(Base, TimestampMixin):
     is_email_verified = Column(Boolean, default=False, nullable=False)
     last_login = Column(DateTime, nullable=True)
 
+    # Language preference (NULL = use context default: vendor dashboard_language or system default)
+    # Supported: en, fr, de, lb
+    preferred_language = Column(String(5), nullable=True)
+
     # Relationships
     marketplace_import_jobs = relationship(
         "MarketplaceImportJob", back_populates="user"