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/Cultures.cs

@@ -0,0 +1,12 @@
+namespace EnvelopeGenerator.GeneratorAPI.Models;
+
+public class Cultures : List<Culture>
+{
+    public IEnumerable<string> Languages => this.Select(c => c.Language);
+
+    public IEnumerable<string> FIClasses => this.Select(c => c.FIClass);
+
+    public Culture Default => this.First();
+    
+    public Culture? this[string? language] => language is null ? null : this.Where(c => c.Language == language).FirstOrDefault();
+}