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/vendor.py

@@ -59,6 +59,20 @@ class VendorCreate(BaseModel):
     business_address: str | None = Field(None, description="Override company business address")
     tax_number: str | None = Field(None, description="Override company tax number")
 
+    # Language Settings
+    default_language: str | None = Field(
+        "fr", description="Default language for content (en, fr, de, lb)"
+    )
+    dashboard_language: str | None = Field(
+        "fr", description="Vendor dashboard UI language"
+    )
+    storefront_language: str | None = Field(
+        "fr", description="Default storefront language for customers"
+    )
+    storefront_languages: list[str] | None = Field(
+        default=["fr", "de", "en"], description="Enabled languages for storefront"
+    )
+
     @field_validator("subdomain")
     @classmethod
     def validate_subdomain(cls, v):
@@ -110,6 +124,20 @@ class VendorUpdate(BaseModel):
         None, description="If true, reset all contact fields to inherit from company"
     )
 
+    # Language Settings
+    default_language: str | None = Field(
+        None, description="Default language for content (en, fr, de, lb)"
+    )
+    dashboard_language: str | None = Field(
+        None, description="Vendor dashboard UI language"
+    )
+    storefront_language: str | None = Field(
+        None, description="Default storefront language for customers"
+    )
+    storefront_languages: list[str] | None = Field(
+        None, description="Enabled languages for storefront"
+    )
+
     @field_validator("subdomain")
     @classmethod
     def subdomain_lowercase(cls, v):
@@ -148,6 +176,12 @@ class VendorResponse(BaseModel):
     is_active: bool
     is_verified: bool
 
+    # Language Settings (optional with defaults for backward compatibility)
+    default_language: str = "fr"
+    dashboard_language: str = "fr"
+    storefront_language: str = "fr"
+    storefront_languages: list[str] = ["fr", "de", "en"]
+
     # Timestamps
     created_at: datetime
     updated_at: datetime