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>

package.json

@@ -11,6 +11,7 @@
   },
   "dependencies": {
     "@hookform/resolvers": "^5.2.2",
+    "@microsoft/signalr": "^9.0.6",
     "@radix-ui/react-dialog": "^1.1.15",
     "@radix-ui/react-dropdown-menu": "^2.1.16",
     "@radix-ui/react-label": "^2.1.7",