Add GuidelineId property to ZugferdInvoice model

Added a new `GuidelineId` property to the `ZugferdInvoice` model to store the ZUGFeRD Guideline ID extracted from XMP metadata. Updated the `AppDbContextModelSnapshot.cs` to reflect this change and created a migration (`20260527133241_AddGuidelineId`) to add the `GuidelineId` column to the `ZugferdInvoices` table.

Generated the corresponding designer file for the migration to define the updated database schema. Minor formatting changes were made to `ZugferdInvoice.cs` without functional impact.

DXApp.TemplateKitProject/Migrations/20260527133241_AddGuidelineId.cs

@@ -0,0 +1,28 @@
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace DXApp.TemplateKitProject.Migrations
+{
+    /// <inheritdoc />
+    public partial class AddGuidelineId : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.AddColumn<string>(
+                name: "GuidelineId",
+                table: "ZugferdInvoices",
+                type: "nvarchar(max)",
+                nullable: true);
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropColumn(
+                name: "GuidelineId",
+                table: "ZugferdInvoices");
+        }
+    }
+}