Introduced the `InvoiceAttachment` entity and its relationship with `ZugferdInvoice` to manage extracted invoice attachments. Updated `AppDbContext` and added a migration to create the `InvoiceAttachments` table with cascading delete behavior and an index for optimized queries.
Enhanced the UI to display attachments in `Details.cshtml`, including file type icons, file size, and extraction date. Added a new `ViewAttachment` page to render or download attachments based on their type, with support for XML, plain text, images, and downloads.
Implemented `AttachmentViewerService` to determine viewer types and MIME types for attachments. Registered the service in the DI container. Updated `Upload.cshtml.cs` to save extracted attachments to the database.
Improved user experience with syntax highlighting for XML files and appropriate messages for unsupported file types.
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.
