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/Index.cshtml

@@ -27,6 +27,16 @@ else
         )
         .Columns(cols =>
         {
+            cols.Add()
+                 .Caption("Details")
+                 .Width(80)
+                 .CellTemplate(new JS(@"function(container, options) {
+                 $('<a>')
+                 .attr('href', '/Invoices/Details?id=' + options.data.Id)
+                 .attr('class', 'btn btn-sm btn-outline-primary')
+                 .text('Details')
+                 .appendTo(container);
+                 }"));
             cols.Add().DataField("Id").Caption("ID").Width(60).AllowEditing(false);
             cols.Add().DataField("InvoiceNumber").Caption("Rechnungsnr.");
             cols.Add().DataField("InvoiceDate").Caption("Datum")