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

@@ -0,0 +1,55 @@
+@page
+@model DXApp.TemplateKitProject.Pages.Invoices.IndexModel
+@{
+    ViewData["Title"] = "Rechnungen";
+}
+
+<h2>Importierte Rechnungen</h2>
+
+@if (!Model.Invoices.Any())
+{
+    <div class="alert alert-info mt-3">Noch keine Rechnungen importiert.</div>
+}
+else
+{
+    @(Html.DevExtreme().DataGrid<DXApp.TemplateKitProject.Models.ZugferdInvoice>()
+        .DataSource(Model.Invoices)
+        .KeyExpr("Id")
+        .ShowBorders(true)
+        .RowAlternationEnabled(true)
+        .FilterRow(f => f.Visible(true))
+        .HeaderFilter(h => h.Visible(true))
+        .Paging(p => p.PageSize(20))
+        .Pager(p => p
+            .ShowPageSizeSelector(true)
+            .AllowedPageSizes(new[] { 10, 20, 50 })
+            .ShowInfo(true)
+        )
+        .Columns(cols =>
+        {
+            cols.Add().DataField("Id").Caption("ID").Width(60).AllowEditing(false);
+            cols.Add().DataField("InvoiceNumber").Caption("Rechnungsnr.");
+            cols.Add().DataField("InvoiceDate").Caption("Datum")
+                .DataType(GridColumnDataType.Date)
+                .Format("dd.MM.yyyy")
+                .Width(110);
+            cols.Add().DataField("SellerName").Caption("Verkäufer");
+            cols.Add().DataField("BuyerName").Caption("Käufer");
+            cols.Add().DataField("TotalAmount").Caption("Gesamt")
+                .DataType(GridColumnDataType.Number)
+                .Format("#,##0.00")
+                .Width(110);
+            cols.Add().DataField("TaxAmount").Caption("MwSt.")
+                .DataType(GridColumnDataType.Number)
+                .Format("#,##0.00")
+                .Width(100);
+            cols.Add().DataField("CurrencyCode").Caption("Währung").Width(80);
+            cols.Add().DataField("Iban").Caption("IBAN");
+            cols.Add().DataField("SourceType").Caption("Quelle").Width(90);
+            cols.Add().DataField("ImportedAt").Caption("Importiert am")
+                .DataType(GridColumnDataType.DateTime)
+                .Format("dd.MM.yyyy HH:mm")
+                .Width(150);
+        })
+    )
+}

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();
+    }
+}