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/AppDbContextModelSnapshot.cs

@@ -36,6 +36,9 @@ namespace DXApp.TemplateKitProject.Migrations
                     b.Property<string>("CurrencyCode")
                         .HasColumnType("nvarchar(max)");
 
+                    b.Property<string>("GuidelineId")
+                        .HasColumnType("nvarchar(max)");
+
                     b.Property<string>("Iban")
                         .HasColumnType("nvarchar(max)");