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

prisma/schema.prisma

@@ -93,6 +93,12 @@ model ShopSettings {
   smtpFromEmail     String   @default("")
   smtpReplyTo       String   @default("")
 
+  // Email templates (HTML, with {{var}} placeholders). Empty = use defaults.
+  emailSubjectDe    String   @default("")
+  emailBodyHtmlDe   String   @default("")
+  emailSubjectEn    String   @default("")
+  emailBodyHtmlEn   String   @default("")
+
   createdAt         DateTime @default(now())
   updatedAt         DateTime @updatedAt