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/Pages/Shared/_Layout.cshtml

@@ -78,6 +78,7 @@
     <script src="~/lib/htmx.min.js"></script>
     <script src="~/lib/Sortable.min.js"></script>
     <script src="~/lib/chart.umd.min.js"></script>
+    <script src="~/lib/signalr.min.js"></script>
     <script src="~/js/app.js"></script>
     @await RenderSectionAsync("Scripts", required: false)
 </body>