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/PlatformContext.tsx

@@ -0,0 +1,7 @@
+import { createContext } from 'react';
+import type { PlatformContext as PlatformContextType } from './types';
+
+// Create the context with undefined default (will be provided by PlatformProvider)
+export const PlatformContext = createContext<PlatformContextType | null>(null);
+
+PlatformContext.displayName = 'PlatformContext';