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/wwwroot/css/site.css

@@ -1281,6 +1281,21 @@ body::before {
     flex-shrink: 0;
 }
 
+@keyframes activity-slide-in {
+    from {
+        opacity: 0;
+        transform: translateY(-8px);
+    }
+    to {
+        opacity: 1;
+        transform: translateY(0);
+    }
+}
+
+.activity-item--new {
+    animation: activity-slide-in 0.3s ease-out;
+}
+
 
 /* ============================================================
    CHART CONTAINER