feat(web): add real-time activity feed via SignalR

- Add ActivityHub and wire up SignalR in Program.cs
- Broadcast new context events from ContextController
- Connect SignalR client on Analytics page for live feed updates
- Restructure activity feed HTML to support live prepending
- Add slide-in animation for new activity items
- Update CORS to allow credentials for SignalR

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

TaskTracker.Api/Hubs/ActivityHub.cs

@@ -0,0 +1,5 @@
+using Microsoft.AspNetCore.SignalR;
+
+namespace TaskTracker.Api.Hubs;
+
+public class ActivityHub : Hub { }