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/platform/PlatformProvider.tsx

@@ -0,0 +1,43 @@
+import { useMemo, type ReactNode } from 'react';
+import { PlatformContext } from '@/platform/PlatformContext';
+import { createTelegramAdapter } from '@/platform/adapters/TelegramAdapter';
+import { createWebAdapter } from '@/platform/adapters/WebAdapter';
+import type { PlatformContext as PlatformContextType } from '@/platform/types';
+
+interface PlatformProviderProps {
+  children: ReactNode;
+}
+
+function detectPlatform(): 'telegram' | 'web' {
+  // Check if Telegram WebApp is available and actually initialized with data
+  // initData is an empty string when not in real Telegram context
+  if (
+    typeof window !== 'undefined' &&
+    window.Telegram?.WebApp?.initData &&
+    window.Telegram.WebApp.initData.length > 0
+  ) {
+    return 'telegram';
+  }
+  return 'web';
+}
+
+function createAdapter(): PlatformContextType {
+  const platform = detectPlatform();
+
+  if (platform === 'telegram') {
+    return createTelegramAdapter();
+  }
+
+  return createWebAdapter();
+}
+
+export function PlatformProvider({ children }: PlatformProviderProps) {
+  // Create adapter once on mount
+  // Using useMemo to ensure stable reference
+  const platformContext = useMemo(() => createAdapter(), []);
+
+  return <PlatformContext.Provider value={platformContext}>{children}</PlatformContext.Provider>;
+}
+
+// Re-export types for convenience
+export type { PlatformContextType as PlatformContext };