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/components/admin/AdminBackButton.tsx

@@ -0,0 +1,33 @@
+import { Link } from 'react-router-dom';
+import { usePlatform } from '@/platform';
+import { BackIcon } from './icons';
+
+interface AdminBackButtonProps {
+  to?: string;
+  className?: string;
+}
+
+/**
+ * Back button for admin pages.
+ * Hidden in Telegram Mini App since native back button is used instead.
+ */
+export function AdminBackButton({ to = '/admin', className }: AdminBackButtonProps) {
+  const { platform } = usePlatform();
+
+  // In Telegram Mini App, we use native back button
+  if (platform === 'telegram') {
+    return null;
+  }
+
+  return (
+    <Link
+      to={to}
+      className={
+        className ||
+        'flex h-10 w-10 items-center justify-center rounded-xl border border-dark-700 bg-dark-800 transition-colors hover:border-dark-600'
+      }
+    >
+      <BackIcon />
+    </Link>
+  );
+}