Add core model classes for auth, culture, images, and links

Introduced new models in EnvelopeGenerator.GeneratorAPI.Models:
- Auth, ContactLink, Culture, Cultures, CustomImages, ErrorViewModel, Image, MainViewModel, and TFARegParams.
These provide foundational structures for authentication, localization, error handling, image management, and contact links. All changes are new file additions.

EnvelopeGenerator.GeneratorAPI/Models/MainViewModel.cs

@@ -0,0 +1,6 @@
+namespace EnvelopeGenerator.GeneratorAPI.Models;
+
+public class MainViewModel
+{
+    public string? Title { get; init; }
+}