feat: Linear-style UI redesign with improved mobile experience

Major changes:
- Redesign cabinet with Linear-style components and top navigation
- Replace detail modals with dedicated pages (users, broadcasts, email templates)
- Add email channel support for broadcasts
- Remove pull-to-refresh, improve drag-and-drop on touch devices
- Fix Telegram Mini App: fullscreen, back button, scroll restoration
- Unify admin pages color scheme to design system
- Mobile-first improvements: horizontal tabs for settings, better touch targets

src/locales/en.json

@@ -28,7 +28,8 @@
       "perGb": "/GB"
     },
     "retry": "Retry",
-    "saving": "Saving..."
+    "saving": "Saving...",
+    "understand": "Got it"
   },
   "nav": {
     "dashboard": "Dashboard",
@@ -761,7 +762,9 @@
       "resetted": "Template reset to default",
       "testSent": "Test email sent",
       "variables": "Available Variables",
-      "clickToCopy": "Click to copy"
+      "clickToCopy": "Click to copy",
+      "previewDesktopOnly": "Preview is only available in the desktop web version",
+      "previewNotAvailable": "Preview not available"
     },
     "payments": {
       "title": "Payment verification",