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/schema/auth.py

@@ -25,6 +25,7 @@ class UserResponse(BaseModel):
     username: str
     role: str
     is_active: bool
+    preferred_language: str | None = None
     last_login: datetime | None = None
     created_at: datetime
     updated_at: datetime
@@ -58,6 +59,9 @@ class UserUpdate(BaseModel):
     role: str | None = Field(None, pattern="^(admin|vendor)$")
     is_active: bool | None = None
     is_email_verified: bool | None = None
+    preferred_language: str | None = Field(
+        None, description="Preferred language (en, fr, de, lb)"
+    )
 
     @field_validator("username")
     @classmethod
@@ -78,6 +82,9 @@ class UserCreate(BaseModel):
     first_name: str | None = Field(None, max_length=100)
     last_name: str | None = Field(None, max_length=100)
     role: str = Field(default="vendor", pattern="^(admin|vendor)$")
+    preferred_language: str | None = Field(
+        None, description="Preferred language (en, fr, de, lb)"
+    )
 
     @field_validator("username")
     @classmethod