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/primitives/Sheet/index.ts

@@ -0,0 +1,19 @@
+export {
+  Sheet,
+  SheetTrigger,
+  SheetPortal,
+  SheetClose,
+  SheetOverlay,
+  SheetContent,
+  SheetHeader,
+  SheetFooter,
+  SheetTitle,
+  SheetDescription,
+  type SheetProps,
+  type SheetOverlayProps,
+  type SheetContentProps,
+  type SheetHeaderProps,
+  type SheetFooterProps,
+  type SheetTitleProps,
+  type SheetDescriptionProps,
+} from './Sheet';