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/Image.cs

@@ -0,0 +1,10 @@
+namespace EnvelopeGenerator.GeneratorAPI.Models;
+
+public class Image
+{
+    public string Src { get; init; } = string.Empty;
+
+    public Dictionary<string, string> Classes { get; init; } = new();
+
+    public string GetClassIn(string page) => Classes.TryGetValue(page, out var cls) && cls is not null ? cls : string.Empty;
+}