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/App.tsx

@@ -135,7 +135,6 @@ const AdminBroadcastDetail = lazyWithRetry(() => import('./pages/AdminBroadcastD
 const AdminPinnedMessages = lazyWithRetry(() => import('./pages/AdminPinnedMessages'));
 const AdminPinnedMessageCreate = lazyWithRetry(() => import('./pages/AdminPinnedMessageCreate'));
 const AdminChannelSubscriptions = lazyWithRetry(() => import('./pages/AdminChannelSubscriptions'));
-const AdminEmailTemplatePreview = lazyWithRetry(() => import('./pages/AdminEmailTemplatePreview'));
 const AdminRoles = lazyWithRetry(() => import('./pages/AdminRoles'));
 const AdminRoleEdit = lazyWithRetry(() => import('./pages/AdminRoleEdit'));
 const AdminRoleAssign = lazyWithRetry(() => import('./pages/AdminRoleAssign'));
@@ -1212,17 +1211,6 @@ function App() {
             </PermissionRoute>
           }
         />
-        <Route
-          path="/admin/email-templates/preview/:type/:lang"
-          element={
-            <PermissionRoute permission="email_templates:read">
-              <LazyPage>
-                <AdminEmailTemplatePreview />
-              </LazyPage>
-            </PermissionRoute>
-          }
-        />
-
         {/* RBAC routes */}
         <Route
           path="/admin/roles"