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/CustomImages.cs

@@ -0,0 +1,6 @@
+namespace EnvelopeGenerator.GeneratorAPI.Models;
+
+public class CustomImages : Dictionary<string, Image>
+{
+    public new Image this[string key] => TryGetValue(key, out var img) && img is not null ? img : new();
+}