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/Culture.cs

@@ -0,0 +1,17 @@
+using System.Globalization;
+
+namespace EnvelopeGenerator.GeneratorAPI.Models;
+
+public class Culture
+{
+    private string _language = string.Empty;
+    public string Language { get => _language;
+        init {
+            _language = value;
+            Info = new(value);
+        } 
+    }
+    public string FIClass { get; init; } = string.Empty;
+
+    public CultureInfo? Info { get; init; }
+}