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/Auth.cs

@@ -0,0 +1,14 @@
+namespace EnvelopeGenerator.GeneratorAPI.Models;
+
+public record Auth(string? AccessCode = null, string? SmsCode = null, string? AuthenticatorCode = null, bool UserSelectSMS = default)
+{
+    public bool HasAccessCode => AccessCode is not null;
+
+    public bool HasSmsCode => SmsCode is not null;
+
+    public bool HasAuthenticatorCode => AuthenticatorCode is not null;
+
+    public bool HasMulti => new[] { HasAccessCode, HasSmsCode, HasAuthenticatorCode }.Count(state => state) > 1;
+
+    public bool HasNone => !(HasAccessCode || HasSmsCode || HasAuthenticatorCode);
+}