Add invoice details page and grid column link

Added a "Details" column to the grid in `Index.cshtml` with a button linking to the new invoice details page. The button dynamically generates the URL using the invoice ID.

Created `Details.cshtml` to display detailed invoice information, including metadata such as seller, buyer, amounts, and dates. Added a back button and error handling for missing invoices.

Introduced `DetailsModel` in `Details.cshtml.cs` to fetch invoice data using `AppDbContext`. Implemented the `OnGetAsync` method to retrieve data asynchronously and handle cases where the invoice is not found.

DXApp.TemplateKitProject/Pages/Invoices/Details.cshtml.cs

@@ -0,0 +1,22 @@
+using DXApp.TemplateKitProject.Data;
+using DXApp.TemplateKitProject.Models;
+using Microsoft.AspNetCore.Mvc;
+using Microsoft.AspNetCore.Mvc.RazorPages;
+using Microsoft.EntityFrameworkCore;
+
+namespace DXApp.TemplateKitProject.Pages.Invoices;
+
+public class DetailsModel(AppDbContext db) : PageModel
+{
+    public ZugferdInvoice? Invoice { get; private set; }
+
+    public async Task<IActionResult> OnGetAsync(int id)
+    {
+        Invoice = await db.ZugferdInvoices.FirstOrDefaultAsync(i => i.Id == id);
+
+        if (Invoice is null)
+            return NotFound();
+
+        return Page();
+    }
+}