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/Program.cs

@@ -37,6 +37,9 @@ builder.Services.AddCors(options =>
     });
 });
 
+// Razor Pages
+builder.Services.AddRazorPages();
+
 var app = builder.Build();
 
 // Auto-migrate on startup in development
@@ -51,8 +54,8 @@ app.UseSwagger();
 app.UseSwaggerUI();
 
 app.UseCors();
-app.UseDefaultFiles();
 app.UseStaticFiles();
+app.MapRazorPages();
 app.MapControllers();
 
 app.Run();