Replaced direct file links with a JavaScript-based viewer
(`openAttachmentViewer`) and added a DevExtreme Popup
(`attachment-viewer-popup`) for displaying attachments in a
modal dialog. The viewer supports PDFs (via PDF.js), XML
(with CodeMirror syntax highlighting), plain text, images,
and provides a fallback for unsupported file types.
Dynamically load CodeMirror for XML files and handle errors
gracefully when loading file content. Added `onAttachmentPopupHiding`
to clear popup content on close.
Updated `ViewAttachmentModel` to return text/XML content
directly for AJAX requests, improving frontend performance
and enabling dynamic content loading.
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.
