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/Services/AttachmentViewerService.cs

@@ -0,0 +1,49 @@
+using DXApp.TemplateKitProject.Models;
+
+namespace DXApp.TemplateKitProject.Services;
+
+public class AttachmentViewerService
+{
+    public AttachmentViewerType DetermineViewerType(string fileName)
+    {
+        var extension = Path.GetExtension(fileName).ToLowerInvariant();
+
+        return extension switch
+        {
+            ".pdf" => AttachmentViewerType.Pdf,
+            ".xml" => AttachmentViewerType.Xml,
+            ".txt" => AttachmentViewerType.Text,
+            ".jpg" or ".jpeg" or ".png" or ".gif" => AttachmentViewerType.Image,
+            ".docx" or ".doc" => AttachmentViewerType.Word,
+            _ => AttachmentViewerType.Download
+        };
+    }
+
+    public string GetMimeType(string fileName)
+    {
+        var extension = Path.GetExtension(fileName).ToLowerInvariant();
+
+        return extension switch
+        {
+            ".pdf" => "application/pdf",
+            ".xml" => "application/xml",
+            ".txt" => "text/plain",
+            ".jpg" or ".jpeg" => "image/jpeg",
+            ".png" => "image/png",
+            ".gif" => "image/gif",
+            ".docx" => "application/vnd.openxmlformats-officedocument.wordprocessingml.document",
+            ".doc" => "application/msword",
+            _ => "application/octet-stream"
+        };
+    }
+}
+
+public enum AttachmentViewerType
+{
+    Pdf,        // PDF.js Viewer
+    Xml,        // Syntax-highlighted XML
+    Text,       // Plain Text
+    Image,      // <img> Tag
+    Word,       // Konvertierung zu PDF (optional)
+    Download    // Nur Download-Button
+}