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/customer.py

@@ -24,6 +24,9 @@ class CustomerRegister(BaseModel):
     last_name: str = Field(..., min_length=1, max_length=100)
     phone: str | None = Field(None, max_length=50)
     marketing_consent: bool = Field(default=False)
+    preferred_language: str | None = Field(
+        None, description="Preferred language (en, fr, de, lb)"
+    )
 
     @field_validator("email")
     @classmethod
@@ -52,6 +55,9 @@ class CustomerUpdate(BaseModel):
     last_name: str | None = Field(None, min_length=1, max_length=100)
     phone: str | None = Field(None, max_length=50)
     marketing_consent: bool | None = None
+    preferred_language: str | None = Field(
+        None, description="Preferred language (en, fr, de, lb)"
+    )
 
     @field_validator("email")
     @classmethod
@@ -76,6 +82,7 @@ class CustomerResponse(BaseModel):
     phone: str | None
     customer_number: str
     marketing_consent: bool
+    preferred_language: str | None
     last_order_date: datetime | None
     total_orders: int
     total_spent: Decimal
@@ -169,7 +176,9 @@ class CustomerPreferencesUpdate(BaseModel):
     """Schema for updating customer preferences."""
 
     marketing_consent: bool | None = None
-    language: str | None = Field(None, max_length=10)
+    preferred_language: str | None = Field(
+        None, description="Preferred language (en, fr, de, lb)"
+    )
     currency: str | None = Field(None, max_length=3)
     notification_preferences: dict[str, bool] | None = None
 
@@ -206,6 +215,7 @@ class CustomerDetailResponse(BaseModel):
     phone: str | None = None
     customer_number: str | None = None
     marketing_consent: bool | None = None
+    preferred_language: str | None = None
     last_order_date: datetime | None = None
     total_orders: int | None = None
     total_spent: Decimal | None = None