feat(email): WYSIWYG template editor with variable substitution

- Add emailSubject{De,En} + emailBodyHtml{De,En} to ShopSettings
- New RichTextEditor component (TipTap) with toolbar + variable insert
- Settings UI: Email templates section per language
- email.server.ts: substitute {{var}} placeholders, fall back to defaults
- Default vars: invoiceNumber, customerName, customerFirstName, orderName,
  totalGross, dueDate, companyName, ownerName

package.json

@@ -33,6 +33,10 @@
     "@shopify/app-bridge-react": "^4.2.4",
     "@shopify/shopify-app-react-router": "^1.1.0",
     "@shopify/shopify-app-session-storage-prisma": "^8.0.0",
+    "@tiptap/extension-link": "^3.23.1",
+    "@tiptap/pm": "^3.23.1",
+    "@tiptap/react": "^3.23.1",
+    "@tiptap/starter-kit": "^3.23.1",
     "@types/nodemailer": "^8.0.0",
     "isbot": "^5.1.31",
     "nodemailer": "^8.0.7",