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/templates/vendor/base.html

@@ -13,6 +13,9 @@
     <!-- Tailwind CSS v4 (built locally via standalone CLI) -->
     <link rel="stylesheet" href="{{ url_for('static', path='vendor/css/tailwind.output.css') }}" />
 
+    <!-- Flag Icons for Language Selector -->
+    <link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/flag-icons@7.2.3/css/flag-icons.min.css" />
+
     <!-- Alpine Cloak -->
     <style>
         [x-cloak] { display: none !important; }