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>

static/admin/js/marketplace-letzshop.js

@@ -218,7 +218,7 @@ function adminMarketplaceLetzshop() {
         },
 
         /**
-         * Load cross-vendor aggregate data
+         * Load cross-vendor aggregate data (when no vendor is selected)
          */
         async loadCrossVendorData() {
             marketplaceLetzshopLog.info('Loading cross-vendor data');
@@ -226,6 +226,7 @@ function adminMarketplaceLetzshop() {
 
             try {
                 await Promise.all([
+                    this.loadProducts(),
                     this.loadOrders(),
                     this.loadExceptions(),
                     this.loadExceptionStats(),
@@ -428,27 +429,25 @@ function adminMarketplaceLetzshop() {
         // ═══════════════════════════════════════════════════════════════
 
         /**
-         * Load Letzshop products for selected vendor
-         * Uses /admin/products endpoint with marketplace=Letzshop filter
+         * Load Letzshop products
+         * When vendor is selected: shows products for that vendor
+         * When no vendor selected: shows ALL Letzshop marketplace products
          */
         async loadProducts() {
-            if (!this.selectedVendor) {
-                this.products = [];
-                this.totalProducts = 0;
-                this.productStats = { total: 0, active: 0, inactive: 0, last_sync: null };
-                return;
-            }
-
             this.loadingProducts = true;
 
             try {
                 const params = new URLSearchParams({
                     marketplace: 'Letzshop',
-                    vendor_name: this.selectedVendor.name,
                     skip: ((this.productsPage - 1) * this.productsLimit).toString(),
                     limit: this.productsLimit.toString()
                 });
 
+                // Filter by vendor if one is selected
+                if (this.selectedVendor) {
+                    params.append('vendor_name', this.selectedVendor.name);
+                }
+
                 if (this.productFilters.search) {
                     params.append('search', this.productFilters.search);
                 }
@@ -474,15 +473,19 @@ function adminMarketplaceLetzshop() {
 
         /**
          * Load product statistics for Letzshop products
+         * Shows stats for selected vendor or all Letzshop products
          */
         async loadProductStats() {
-            if (!this.selectedVendor) return;
-
             try {
                 const params = new URLSearchParams({
-                    marketplace: 'Letzshop',
-                    vendor_name: this.selectedVendor.name
+                    marketplace: 'Letzshop'
                 });
+
+                // Filter by vendor if one is selected
+                if (this.selectedVendor) {
+                    params.append('vendor_name', this.selectedVendor.name);
+                }
+
                 const response = await apiClient.get(`/admin/products/stats?${params}`);
                 this.productStats = {
                     total: response.total || 0,