feat: show all Letzshop products in Products tab without vendor filter

- Update loadProducts() to show all Letzshop marketplace products when
  no vendor is selected, filtered by vendor when one is selected
- Add vendor column to products table (shown when no vendor filter)
- Update header to show context-aware subtitle
- Hide Import/Export buttons when no vendor selected
- Remove "Marketplace Products" from sidebar menu (accessible via
  Marketplace > Letzshop > Products tab)

🤖 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

@@ -78,7 +78,6 @@
     <!-- Vendor Operations Section -->
     {{ section_header('Vendor Operations', 'vendorOps') }}
     {% call section_content('vendorOps') %}
-        {{ menu_item('marketplace-products', '/admin/marketplace-products', 'database', 'Marketplace Products') }}
         {{ menu_item('vendor-products', '/admin/vendor-products', 'cube', 'Vendor Products') }}
         {{ menu_item('customers', '/admin/customers', 'user-group', 'Customers') }}
         {{ menu_item('inventory', '/admin/inventory', 'archive', 'Inventory') }}