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>

mkdocs.yml

@@ -31,6 +31,7 @@ nav:
     - Overview: architecture/overview.md
     - Marketplace Integration: architecture/marketplace-integration.md
     - Architecture Patterns: architecture/architecture-patterns.md
+    - Language & i18n: architecture/language-i18n.md
     - Company-Vendor Management: architecture/company-vendor-management.md
     - Multi-Tenant System: architecture/multi-tenant.md
     - Middleware Stack: architecture/middleware.md
@@ -124,6 +125,7 @@ nav:
       - Quick Summary: development/customer-auth-summary.md
     - Migrations:
       - Database Migrations: development/migration/database-migrations.md
+      - Language & i18n Implementation: development/migration/language-i18n-implementation.md
       - Tailwind CSS Migration: development/migration/tailwind-migration-plan.md
       - Makefile Refactoring: development/migration/makefile-refactoring-complete.md
       - SVC-006 Migration Plan: development/migration/svc-006-migration-plan.md
@@ -186,7 +188,9 @@ nav:
     - Shop Setup: guides/shop-setup.md
     - CSV Import: guides/csv-import.md
     - Marketplace Integration: guides/marketplace-integration.md
-    - Letzshop Order Integration: guides/letzshop-order-integration.md
+    - Letzshop:
+      - Order Integration: guides/letzshop-order-integration.md
+      - Marketplace API: guides/letzshop-marketplace-api.md
 
   # ============================================
   # TROUBLESHOOTING