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/Shared/_Layout.cshtml

@@ -84,6 +84,18 @@
                         .Icon("info")
                         .Option("path", GetUrl("/About"))
                         .Selected(IsCurrentUrl("/About"));
+
+                    items.Add()
+                        .Text("Rechnungen")
+                        .Icon("doc")
+                        .Option("path", GetUrl("/Invoices/Index"))
+                        .Selected(IsCurrentUrl("/Invoices/Index"));
+
+                    items.Add()
+                        .Text("PDF hochladen")
+                        .Icon("upload")
+                        .Option("path", GetUrl("/Invoices/Upload"))
+                        .Selected(IsCurrentUrl("/Invoices/Upload"));
                 })
                 .ExpandEvent(TreeViewExpandEvent.Click)
                 .SelectionMode(NavSelectionMode.Single)