fix: Add variable dropdown to TipTap rich text editor

Fixed missing variable dropdown in email template editor.

Problem:
- RichTextEditor component had dropdown functionality
- But variables prop was empty array
- Users had to manually type {variable_name}

Solution:
- Added comprehensive list of 40+ available variables
- Includes order, customer, payment, shipping, URL, store variables
- Variables now show in dropdown for easy insertion

Available Variables:
- Order: order_number, order_total, order_items_table, etc.
- Customer: customer_name, customer_email, customer_phone
- Payment: payment_method, transaction_id, payment_retry_url
- Shipping: tracking_number, tracking_url, shipping_carrier
- URLs: order_url, review_url, shop_url, my_account_url
- Store: site_name, support_email, current_year

Now users can click dropdown and select variables instead of typing them manually.

admin-spa/src/routes/Settings/Notifications/EditTemplate.tsx

@@ -36,8 +36,55 @@ export default function EditTemplate() {
   const [subject, setSubject] = useState('');
   const [markdownContent, setMarkdownContent] = useState(''); // Source of truth: Markdown
   const [blocks, setBlocks] = useState<EmailBlock[]>([]); // Visual mode view (derived from markdown)
-  const [variables, setVariables] = useState<{ [key: string]: string }>({});
   const [activeTab, setActiveTab] = useState('preview');
+  
+  // All available template variables
+  const availableVariables = [
+    // Order variables
+    'order_number',
+    'order_id',
+    'order_date',
+    'order_total',
+    'order_subtotal',
+    'order_tax',
+    'order_shipping',
+    'order_discount',
+    'order_status',
+    'order_url',
+    'order_items_table',
+    'completion_date',
+    'estimated_delivery',
+    // Customer variables
+    'customer_name',
+    'customer_first_name',
+    'customer_last_name',
+    'customer_email',
+    'customer_phone',
+    'billing_address',
+    'shipping_address',
+    // Payment variables
+    'payment_method',
+    'payment_status',
+    'payment_date',
+    'transaction_id',
+    'payment_retry_url',
+    // Shipping/Tracking variables
+    'tracking_number',
+    'tracking_url',
+    'shipping_carrier',
+    'shipping_method',
+    // URL variables
+    'review_url',
+    'shop_url',
+    'my_account_url',
+    // Store variables
+    'site_name',
+    'site_title',
+    'store_name',
+    'store_url',
+    'support_email',
+    'current_year',
+  ];
 
   // Fetch email customization settings
   const { data: emailSettings } = useQuery({
@@ -129,8 +176,8 @@ export default function EditTemplate() {
     setBlocks(newBlocks); // Keep blocks in sync
   };
 
-  // Get variable keys for the rich text editor
-  const variableKeys = Object.keys(variables);
+  // Variable keys for the rich text editor dropdown
+  const variableKeys = availableVariables;
   
   // Parse [card] tags and [button] shortcodes for preview
   const parseCardsForPreview = (content: string) => {