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/index.ts

@@ -0,0 +1,33 @@
+// Platform abstraction layer
+// Provides unified APIs for Telegram Mini Apps and web browser
+
+// Context and Provider
+export { PlatformContext } from './PlatformContext';
+export { PlatformProvider } from './PlatformProvider';
+
+// Types
+export type {
+  PlatformType,
+  PlatformContext as PlatformContextType,
+  PlatformCapabilities,
+  MainButtonConfig,
+  PopupOptions,
+  PopupButton,
+  InvoiceStatus,
+  HapticImpactStyle,
+  HapticNotificationType,
+  BackButtonController,
+  MainButtonController,
+  HapticController,
+  DialogController,
+  ThemeController,
+  CloudStorageController,
+  TelegramThemeParams,
+} from './types';
+
+// Hooks
+export { usePlatform, useIsTelegram, useCapability } from './hooks/usePlatform';
+export { useBackButton, useConditionalBackButton } from './hooks/useBackButton';
+export { useMainButton, useSimpleMainButton } from './hooks/useMainButton';
+export { useHaptic, useHapticClick, useHapticFeedback } from './hooks/useHaptic';
+export { useNativeDialog, useDestructiveConfirm, PopupButtons } from './hooks/useNativeDialog';