Added indexes for `ImportedAt` and a composite key on
`InvoiceNumber` and `SellerTaxId` to improve query performance.
Updated `AppDbContext` and EF Core migrations to reflect these
changes. Introduced `ZugferdInvoiceListDto` to optimize memory
usage by excluding large fields like `RawXml`. Updated the
frontend (`Index.cshtml`) and backend (`Index.cshtml.cs`) to use
the new DTO for better performance.
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`.
