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>

main.py

@@ -39,6 +39,7 @@ from app.exceptions.handler import setup_exception_handlers
 # Import page routers
 from app.routes import admin_pages, shop_pages, vendor_pages
 from middleware.context import ContextMiddleware
+from middleware.language import LanguageMiddleware
 from middleware.logging import LoggingMiddleware
 from middleware.theme_context import ThemeContextMiddleware
 
@@ -89,13 +90,15 @@ app.add_middleware(
 # Desired execution order:
 #   1. VendorContextMiddleware    (detect vendor, extract clean_path)
 #   2. ContextMiddleware          (detect context using clean_path)
-#   3. ThemeContextMiddleware     (load theme)
-#   4. LoggingMiddleware          (log all requests)
+#   3. LanguageMiddleware         (detect language based on context)
+#   4. ThemeContextMiddleware     (load theme)
+#   5. LoggingMiddleware          (log all requests)
 #
 # Therefore we add them in REVERSE:
 #   - Add ThemeContextMiddleware FIRST (runs LAST in request)
-#   - Add ContextMiddleware SECOND
-#   - Add VendorContextMiddleware THIRD
+#   - Add LanguageMiddleware SECOND (runs after context)
+#   - Add ContextMiddleware THIRD
+#   - Add VendorContextMiddleware FOURTH
 #   - Add LoggingMiddleware LAST (runs FIRST for timing)
 # ============================================================================
 
@@ -111,6 +114,10 @@ app.add_middleware(LoggingMiddleware)
 logger.info("Adding ThemeContextMiddleware (detects and loads theme)")
 app.add_middleware(ThemeContextMiddleware)
 
+# Add language middleware (detects language after context is determined)
+logger.info("Adding LanguageMiddleware (detects language based on context)")
+app.add_middleware(LanguageMiddleware)
+
 # Add context detection middleware (runs after vendor context extraction)
 logger.info("Adding ContextMiddleware (detects context type using clean_path)")
 app.add_middleware(ContextMiddleware)
@@ -125,8 +132,9 @@ logger.info("  Execution order (request →):")
 logger.info("    1. LoggingMiddleware (timing)")
 logger.info("    2. VendorContextMiddleware (vendor detection)")
 logger.info("    3. ContextMiddleware (context detection)")
-logger.info("    4. ThemeContextMiddleware (theme loading)")
-logger.info("    5. FastAPI Router")
+logger.info("    4. LanguageMiddleware (language detection)")
+logger.info("    5. ThemeContextMiddleware (theme loading)")
+logger.info("    6. FastAPI Router")
 logger.info("=" * 80)
 
 # ========================================