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>

lib/signalr/config.ts

@@ -0,0 +1,13 @@
+export const SIGNALR_CONFIG = {
+  HUB_URLS: {
+    PROJECT: `${process.env.NEXT_PUBLIC_API_URL}/hubs/project`,
+    NOTIFICATION: `${process.env.NEXT_PUBLIC_API_URL}/hubs/notification`,
+  },
+
+  // 重连配置
+  RECONNECT_DELAYS: [0, 2000, 5000, 10000, 30000] as number[], // 0s, 2s, 5s, 10s, 30s
+
+  // 日志级别
+  LOG_LEVEL:
+    process.env.NODE_ENV === 'production' ? 'Warning' : 'Information',
+};