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/Pages/Invoices/Details.cshtml.cs

@@ -12,7 +12,9 @@ public class DetailsModel(AppDbContext db) : PageModel
 
     public async Task<IActionResult> OnGetAsync(int id)
     {
-        Invoice = await db.ZugferdInvoices.FirstOrDefaultAsync(i => i.Id == id);
+        Invoice = await db.ZugferdInvoices
+            .Include(i => i.Attachments)
+            .FirstOrDefaultAsync(i => i.Id == id);
 
         if (Invoice is null)
             return NotFound();