fix(web): address code review findings from Razor Pages migration

- Remove legacy wwwroot files (old index.html, app.css, api.js, page scripts)
- Add Index page that redirects / to /board
- Fix mapping delete button missing handler=Delete in URL
- Add [IgnoreAntiforgeryToken] to AnalyticsModel for consistency
- Remove duplicate JS from _TaskDetail.cshtml (already in app.js)

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

TaskTracker.Api/Pages/Index.cshtml.cs

@@ -0,0 +1,9 @@
+using Microsoft.AspNetCore.Mvc;
+using Microsoft.AspNetCore.Mvc.RazorPages;
+
+namespace TaskTracker.Api.Pages;
+
+public class IndexModel : PageModel
+{
+    public IActionResult OnGet() => RedirectToPage("/Board");
+}