refactor: update terminology from 'rejected' to 'declined'

- Change UI labels to use 'Declined' (matching Letzshop terminology)
- Rename rejectOrder() to declineOrder() in JavaScript
- Add Declined stats card to orders dashboard
- Keep internal sync_status value as 'rejected' for backwards compatibility

🤖 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

@@ -518,20 +518,20 @@ function adminMarketplaceLetzshop() {
         },
 
         /**
-         * Reject an order
+         * Decline an order (all items)
          */
-        async rejectOrder(order) {
+        async declineOrder(order) {
             if (!this.selectedVendor) return;
 
-            if (!confirm('Are you sure you want to reject this order?')) return;
+            if (!confirm('Are you sure you want to decline this order? All items will be marked as unavailable.')) return;
 
             try {
                 await apiClient.post(`/admin/letzshop/vendors/${this.selectedVendor.id}/orders/${order.id}/reject`);
-                this.successMessage = 'Order rejected';
+                this.successMessage = 'Order declined';
                 await this.loadOrders();
             } catch (error) {
-                marketplaceLetzshopLog.error('Failed to reject order:', error);
-                this.error = error.message || 'Failed to reject order';
+                marketplaceLetzshopLog.error('Failed to decline order:', error);
+                this.error = error.message || 'Failed to decline order';
             }
         },