feat(web): add Razor Pages scaffolding to API project

Register Razor Pages services and middleware in Program.cs, removing
UseDefaultFiles() since Razor Pages handles routing. Add shared layout
with dark-themed app shell (header, nav links, search/new-task buttons)
and vendor script references for htmx, Sortable, and Chart.js.

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

TaskTracker.Api/Pages/_ViewStart.cshtml

@@ -0,0 +1,3 @@
+@{
+    Layout = "_Layout";
+}