feat: Add mobile responsive layout, open room access, and admin room management

Mobile Responsive Layout:
- Add useMediaQuery, useIsMobile, useIsTablet, useIsDesktop hooks for device detection
- Create MobileHeader component with hamburger menu and action drawer
- Create BottomToolbar for mobile navigation (Files, Members)
- Create SlidePanel component for full-screen mobile sidebars
- Update RoomDetail.tsx with mobile/desktop conditional rendering
- Update RoomList.tsx with single-column grid and touch-friendly buttons
- Add CSS custom properties for safe areas and touch targets (min 44px)
- Add mobile viewport meta tags for notched devices

Open Room Access:
- All authenticated users can view all rooms (not just their own)
- Users can join active rooms they're not members of
- Add is_member field to room responses
- Update room list API to return all rooms by default

Admin Room Management:
- Add permanent delete functionality for system admins
- Add delete confirmation dialog with room title verification
- Broadcast room deletion via WebSocket to connected users
- Add users search API for adding members

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

frontend/src/hooks/useUsers.ts

@@ -0,0 +1,11 @@
+import { useQuery } from '@tanstack/react-query'
+import { usersService } from '../services/users'
+
+export function useUserSearch(query: string, enabled: boolean = true) {
+  return useQuery({
+    queryKey: ['users', 'search', query],
+    queryFn: () => usersService.searchUsers(query),
+    enabled: enabled && query.length >= 1,
+    staleTime: 1000 * 60, // 1 minute
+  })
+}