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>

static/admin/js/marketplace-product-detail.js

@@ -179,24 +179,24 @@ function adminMarketplaceProductDetail() {
         },
 
         /**
-         * Get full language name from ISO code
+         * Get full language name from ISO code (native names for Luxembourg languages)
          */
         getLanguageName(code) {
             const languages = {
                 'en': 'English',
-                'de': 'German',
-                'fr': 'French',
-                'lb': 'Luxembourgish',
-                'es': 'Spanish',
-                'it': 'Italian',
-                'nl': 'Dutch',
-                'pt': 'Portuguese',
-                'pl': 'Polish',
-                'cs': 'Czech',
-                'da': 'Danish',
-                'sv': 'Swedish',
-                'fi': 'Finnish',
-                'no': 'Norwegian',
+                'de': 'Deutsch',
+                'fr': 'Français',
+                'lb': 'Lëtzebuergesch',
+                'es': 'Español',
+                'it': 'Italiano',
+                'nl': 'Nederlands',
+                'pt': 'Português',
+                'pl': 'Polski',
+                'cs': 'Čeština',
+                'da': 'Dansk',
+                'sv': 'Svenska',
+                'fi': 'Suomi',
+                'no': 'Norsk',
                 'hu': 'Hungarian',
                 'ro': 'Romanian',
                 'bg': 'Bulgarian',