Handle UTF-8 BOM in string conversion

Previously, the method directly converted a byte array to a string
without accounting for a potential UTF-8 Byte Order Mark (BOM).
This commit introduces logic to remove the BOM (if present)
using `TrimStart('\uFEFF')` after converting the byte array
to a string.

Additionally, a comment was added to clarify the purpose of
this change.

DXApp.TemplateKitProject/Services/ZugferdExtractorService.cs

@@ -25,7 +25,9 @@
                 if (isZugferd || attachment.MimeType == "text/xml")
                 {
                     byte[] data = attachment.Data;
-                    return Encoding.UTF8.GetString(data);
+                    // BOM entfernen falls vorhanden (EF BB BF am Anfang)
+                    var text = Encoding.UTF8.GetString(data);
+                    return text.TrimStart('\uFEFF');
                 }
             }