feat(frontend): Implement SignalR client integration for real-time notifications

Add comprehensive SignalR client implementation with connection management,
React hooks, and UI components for real-time notifications and project updates.

Changes:
- Install @microsoft/signalr package (v9.0.6)
- Create SignalR connection manager with auto-reconnect
- Implement useNotificationHub hook for notification hub
- Implement useProjectHub hook for project hub with room-based subscriptions
- Add NotificationPopover UI component with badge and dropdown
- Create Badge UI component
- Add SignalRProvider for global connection initialization
- Update Header to display real-time notifications
- Update app layout to include SignalRProvider
- Add comprehensive documentation in SIGNALR_INTEGRATION.md

Features:
- JWT authentication with automatic token management
- Auto-reconnect with exponential backoff (0s, 2s, 5s, 10s, 30s)
- Connection state management and indicators
- Real-time notification push
- Project event subscriptions (create, update, delete, status change)
- Room-based project subscriptions
- Typing indicators support

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

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

components/layout/Header.tsx

@@ -1,6 +1,6 @@
 'use client';
 
-import { Menu, Bell, LogOut, User } from 'lucide-react';
+import { Menu, LogOut, User } from 'lucide-react';
 import { Button } from '@/components/ui/button';
 import { useUIStore } from '@/stores/ui-store';
 import { useLogout } from '@/lib/hooks/useAuth';
@@ -13,6 +13,7 @@ import {
   DropdownMenuSeparator,
   DropdownMenuTrigger,
 } from '@/components/ui/dropdown-menu';
+import { NotificationPopover } from '@/components/notifications/NotificationPopover';
 
 export function Header() {
   const toggleSidebar = useUIStore((state) => state.toggleSidebar);
@@ -37,10 +38,7 @@ export function Header() {
         </div>
 
         <div className="ml-auto flex items-center gap-4">
-          <Button variant="ghost" size="icon">
-            <Bell className="h-5 w-5" />
-            <span className="sr-only">Notifications</span>
-          </Button>
+          <NotificationPopover />
 
           <DropdownMenu>
             <DropdownMenuTrigger asChild>