Add support for invoice attachments

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.

DXApp.TemplateKitProject/Models/ZugferdInvoice.cs

@@ -17,5 +17,8 @@
         public string SourceType { get; set; } = string.Empty;      // "Upload" oder "Email"
         public string ResultFilePath { get; set; } = string.Empty;  // Pfad der Result-PDF
         public string GuidelineId { get; set; } = string.Empty;     // ZUGFeRD Guideline-ID aus XMP
+
+        // Navigation Property
+        public List<InvoiceAttachment> Attachments { get; set; } = [];
     }
 }