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

@@ -2,6 +2,13 @@
 @model DXApp.TemplateKitProject.Pages.Invoices.DetailsModel
 @{
     ViewData["Title"] = "Rechnungsdetails";
+
+    string FormatFileSize(long bytes)
+    {
+        if (bytes < 1024) return $"{bytes} Bytes";
+        if (bytes < 1024 * 1024) return $"{bytes / 1024.0:N2} KB";
+        return $"{bytes / (1024.0 * 1024.0):N2} MB";
+    }
 }
 
 <h2>📄 Rechnungsdetails</h2>
@@ -11,7 +18,7 @@
 {
     <button class="btn btn-primary mb-3 ms-2"
             onclick="openPdfViewer(@Model.Invoice.Id)">
-        📄 PDF anzeigen
+        📄 Ergebnis anzeigen
     </button>
 }
 
@@ -50,6 +57,51 @@ else
         </tr>
     </table>
 
+    @* Anhänge-Sektion *@
+    @if (Model.Invoice.Attachments.Any())
+    {
+        <h4 class="mt-4">📎 Anhänge (@Model.Invoice.Attachments.Count)</h4>
+        <div class="list-group">
+            @foreach (var attachment in Model.Invoice.Attachments)
+            {
+                var icon = attachment.IsZugferdXml ? "📋" : "📄";
+                var extension = System.IO.Path.GetExtension(attachment.OriginalFileName).ToLowerInvariant();
+                icon = extension switch
+                {
+                    ".xml" => "📋",
+                    ".pdf" => "📄",
+                    ".jpg" or ".jpeg" or ".png" or ".gif" => "🖼️",
+                    ".txt" => "📝",
+                    _ => "📎"
+                };
+
+                <a href="/Invoices/ViewAttachment?filePath=@Uri.EscapeDataString(attachment.SavedFilePath)" 
+                   class="list-group-item list-group-item-action d-flex justify-content-between align-items-center"
+                   target="_blank">
+                    <div>
+                        <span class="me-2">@icon</span>
+                        <strong>@attachment.OriginalFileName</strong>
+                        @if (attachment.IsZugferdXml)
+                        {
+                            <span class="badge bg-success ms-2">ZUGFeRD-XML</span>
+                        }
+                        <br />
+                        <small class="text-muted">
+                            Größe: @FormatFileSize(attachment.FileSizeBytes) · 
+                            Extrahiert: @attachment.ExtractedAt.ToString("dd.MM.yyyy HH:mm")
+                        </small>
+                    </div>
+                    <span class="badge bg-primary">Öffnen</span>
+                </a>
+            }
+        </div>
+    }
+    else
+    {
+        <h4 class="mt-4">📎 Anhänge</h4>
+        <div class="alert alert-info">Keine Anhänge extrahiert.</div>
+    }
+
 @(Html.DevExtreme().Popup()
     .ID("pdf-viewer-popup")
     .Title("PDF Viewer")