fix(admin): rework email template editor (#667043)

Companion to the bot-side fix: defaults now arrive with {placeholders}
instead of baked-in sample values (example.com links).

- inline preview tab (iframe srcDoc) — works on mobile and in Telegram;
  the separate desktop-only preview page and its route are removed
- preview/test render the CURRENT editor content with sample values
- test email: recipient input + detailed backend error messages
- variable chips insert at cursor instead of copying to clipboard
- 'insert default template' button to migrate broken saved overrides
- fa added to language tabs

src/locales/en.json

@@ -1541,9 +1541,13 @@
       "resetted": "Template reset to default",
       "testSent": "Test email sent",
       "variables": "Available Variables",
-      "clickToCopy": "Click to copy",
-      "previewDesktopOnly": "Preview is only available in the desktop web version",
-      "previewNotAvailable": "Preview not available"
+      "editorTab": "Editor",
+      "previewHint": "The preview shows the email with sample values in place of variables.",
+      "clickToInsert": "Click to insert into the text",
+      "insertDefault": "Insert default template",
+      "insertDefaultConfirm": "Replace the current content with the default template?",
+      "testRecipientPlaceholder": "Recipient email (defaults to yours)",
+      "testHint": "Sends the current editor content with sample values in place of variables."
     },
     "payments": {
       "title": "Payments",