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.
