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/api/adminEmailTemplates.ts

@@ -52,6 +52,9 @@ export interface EmailTemplatePreviewResponse {
 export interface EmailTemplateSendTestRequest {
   language: string;
   email?: string;
+  /** Current editor content — when set, the test sends it instead of the saved template */
+  subject?: string;
+  body_html?: string;
 }
 
 export const adminEmailTemplatesApi = {