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>

app/api/main.py

@@ -11,6 +11,7 @@ This module provides:
 from fastapi import APIRouter
 
 from app.api.v1 import admin, shop, vendor
+from app.api.v1.shared import language
 
 api_router = APIRouter()
 
@@ -34,3 +35,10 @@ api_router.include_router(vendor.router, prefix="/v1/vendor", tags=["vendor"])
 # ============================================================================
 
 api_router.include_router(shop.router, prefix="/v1/shop", tags=["shop"])
+
+# ============================================================================
+# SHARED ROUTES (Cross-context utilities)
+# Prefix: /api/v1
+# ============================================================================
+
+api_router.include_router(language.router, prefix="/v1", tags=["language"])