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/TFARegParams.cs

@@ -0,0 +1,17 @@
+namespace EnvelopeGenerator.GeneratorAPI.Models;
+
+/// <summary>
+/// Represents the parameters for two-factor authentication (2FA) registration.
+/// </summary>
+public class TFARegParams
+{
+    /// <summary>
+    /// The maximum allowed time for completing the registration process.
+    /// </summary>
+    public TimeSpan TimeLimit { get; init; } = new(0, 30, 0);
+
+    /// <summary>
+    /// The deadline for registration, calculated as the current time plus the <see cref="TimeLimit"/>.
+    /// </summary>
+    public DateTime Deadline => DateTime.Now.AddTicks(TimeLimit.Ticks);
+}