Adds customers page and related styling

Introduces new customers management interface with data table
Adds customers-specific CSS and updates layout to include new stylesheet
Configures menu service to point to new customers page route

PlanTempus.Application/Features/Customers/Pages/Index.cshtml.cs

@@ -0,0 +1,10 @@
+using Microsoft.AspNetCore.Mvc.RazorPages;
+
+namespace PlanTempus.Application.Features.Customers.Pages;
+
+public class IndexModel : PageModel
+{
+    public void OnGet()
+    {
+    }
+}