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

@@ -0,0 +1,44 @@
+@page
+@model DXApp.TemplateKitProject.Pages.Invoices.DetailsModel
+@{
+    ViewData["Title"] = "Rechnungsdetails";
+}
+
+<h2>📄 Rechnungsdetails</h2>
+<a href="/Invoices" class="btn btn-secondary mb-3">← Zurück zur Liste</a>
+
+@if (Model.Invoice is null)
+{
+    <div class="alert alert-danger">Rechnung nicht gefunden.</div>
+}
+else
+{
+    <table class="table table-sm table-bordered w-auto">
+        <tr><th>ID</th><td>@Model.Invoice.Id</td></tr>
+        <tr><th>Rechnungsnummer</th><td>@Model.Invoice.InvoiceNumber</td></tr>
+        <tr><th>Rechnungsdatum</th><td>@Model.Invoice.InvoiceDate.ToString("dd.MM.yyyy")</td></tr>
+        <tr><th>Verkäufer</th><td>@Model.Invoice.SellerName</td></tr>
+        <tr><th>USt-ID Verkäufer</th><td>@Model.Invoice.SellerTaxId</td></tr>
+        <tr><th>Käufer</th><td>@Model.Invoice.BuyerName</td></tr>
+        <tr><th>Währung</th><td>@Model.Invoice.CurrencyCode</td></tr>
+        <tr><th>Steuerbetrag</th><td>@Model.Invoice.TaxAmount.ToString("N2")</td></tr>
+        <tr><th>Gesamtbetrag</th><td><strong>@Model.Invoice.TotalAmount.ToString("N2")</strong></td></tr>
+        <tr><th>IBAN</th><td>@Model.Invoice.Iban</td></tr>
+        <tr><th>Quelle</th><td>@Model.Invoice.SourceType</td></tr>
+        <tr><th>Guideline-ID</th><td><code>@Model.Invoice.GuidelineId</code></td></tr>
+        <tr><th>Importiert am</th><td>@Model.Invoice.ImportedAt.ToString("dd.MM.yyyy HH:mm")</td></tr>
+        <tr>
+            <th>Result-PDF</th>
+            <td>
+                @if (!string.IsNullOrEmpty(Model.Invoice.ResultFilePath))
+                {
+                    <small class="text-muted">@Model.Invoice.ResultFilePath</small>
+                }
+                else
+                {
+                    <span class="text-muted">– nicht vorhanden –</span>
+                }
+            </td>
+        </tr>
+    </table>
+}