Add invoices page and navigation menu items

Added new navigation menu items in `_Layout.cshtml` for "Rechnungen" (Invoices) and "PDF hochladen" (Upload PDF) with icons, paths, and selection logic.

Created a new Razor Page `Index.cshtml` to display imported invoices using a `DataGrid` with features like filtering, paging, and column formatting. Added a conditional message for when no invoices are available.

Implemented the `IndexModel` class in `Index.cshtml.cs` to handle backend logic, including fetching and sorting invoices from the database using `AppDbContext`.

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

@@ -0,0 +1,18 @@
+using DXApp.TemplateKitProject.Data;
+using DXApp.TemplateKitProject.Models;
+using Microsoft.AspNetCore.Mvc.RazorPages;
+using Microsoft.EntityFrameworkCore;
+
+namespace DXApp.TemplateKitProject.Pages.Invoices;
+
+public class IndexModel(AppDbContext db) : PageModel
+{
+    public List<ZugferdInvoice> Invoices { get; private set; } = [];
+
+    public async Task OnGetAsync()
+    {
+        Invoices = await db.ZugferdInvoices
+            .OrderByDescending(i => i.ImportedAt)
+            .ToListAsync();
+    }
+}