feat: add Letzshop frontend for admin and vendor portals

Add complete frontend UI for Letzshop marketplace integration:

Admin portal (/admin/letzshop):
- Vendor overview with Letzshop status cards
- Vendor table with configuration state and sync info
- Configuration modal for API credentials
- Connection testing and manual sync triggers
- Orders modal for viewing vendor orders

Vendor portal (/vendor/{code}/letzshop):
- Orders tab with import, confirm, reject actions
- Settings tab for API credentials management
- Tracking modal for shipment updates
- Order details modal with line items
- Stats display for order status counts

Also includes:
- Routes for both admin and vendor Letzshop pages
- Sidebar navigation updates for both portals
- Alpine.js data functions for reactive UI state

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/templates/admin/partials/sidebar.html

@@ -81,6 +81,7 @@
         {{ menu_item('marketplace-products', '/admin/marketplace-products', 'database', 'Marketplace Products') }}
         {{ menu_item('vendor-products', '/admin/vendor-products', 'cube', 'Vendor Products') }}
         {{ menu_item('marketplace', '/admin/marketplace', 'cloud-download', 'Import') }}
+        {{ menu_item('letzshop', '/admin/letzshop', 'shopping-cart', 'Letzshop Orders') }}
     {% endcall %}
 
     <!-- Content Management Section -->