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/ErrorViewModel.cs

@@ -0,0 +1,10 @@
+namespace EnvelopeGenerator.GeneratorAPI.Models;
+
+public class ErrorViewModel
+{
+    public string Title { get; init; } = "404";
+
+    public string Subtitle { get; init; } = "Hmmm...";
+
+    public string Body { get; init; } = "It looks like one of the developers fell asleep";
+}