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

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+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);
+}

EnvelopeGenerator.GeneratorAPI/Models/ContactLink.cs

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+namespace EnvelopeGenerator.GeneratorAPI.Models
+{
+    /// <summary>
+    /// Represents a hyperlink for contact purposes with various HTML attributes.
+    /// </summary>
+    public class ContactLink
+    {
+        /// <summary>
+        /// Gets or sets the label of the hyperlink.
+        /// </summary>
+        public string Label { get; init; } = "Contact";
+
+        /// <summary>
+        /// Gets or sets the URL that the hyperlink points to.
+        /// </summary>
+        public string Href { get; set; } = string.Empty;
+
+        /// <summary>
+        /// Gets or sets the target where the hyperlink should open.
+        /// Commonly used values are "_blank", "_self", "_parent", "_top".
+        /// </summary>
+        public string Target { get; set; } = "_blank";
+
+        /// <summary>
+        /// Gets or sets the relationship of the linked URL as space-separated link types.
+        /// Examples include "nofollow", "noopener", "noreferrer".
+        /// </summary>
+        public string Rel { get; set; } = string.Empty;
+
+        /// <summary>
+        /// Gets or sets the filename that should be downloaded when clicking the hyperlink.
+        /// This attribute will only have an effect if the href attribute is set.
+        /// </summary>
+        public string Download { get; set; } = string.Empty;
+
+        /// <summary>
+        /// Gets or sets the language of the linked resource. Useful when linking to
+        /// content in another language.
+        /// </summary>
+        public string HrefLang { get; set; } = "en";
+
+        /// <summary>
+        /// Gets or sets the MIME type of the linked URL. Helps browsers to handle
+        /// the type correctly when the link is clicked.
+        /// </summary>
+        public string Type { get; set; } = string.Empty;
+
+        /// <summary>
+        /// Gets or sets additional information about the hyperlink, typically viewed
+        /// as a tooltip when the mouse hovers over the link.
+        /// </summary>
+        public string Title { get; set; } = string.Empty;
+
+        /// <summary>
+        /// Gets or sets an identifier for the hyperlink, unique within the HTML document.
+        /// </summary>
+        public string Id { get; set; } = string.Empty;
+    }
+
+}

EnvelopeGenerator.GeneratorAPI/Models/Culture.cs

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+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; }
+}

EnvelopeGenerator.GeneratorAPI/Models/Cultures.cs

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+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();
+}

EnvelopeGenerator.GeneratorAPI/Models/CustomImages.cs

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+namespace EnvelopeGenerator.GeneratorAPI.Models;
+
+public class CustomImages : Dictionary<string, Image>
+{
+    public new Image this[string key] => TryGetValue(key, out var img) && img is not null ? img : new();
+}

EnvelopeGenerator.GeneratorAPI/Models/ErrorViewModel.cs

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+namespace EnvelopeGenerator.GeneratorAPI.Models;
+
+public class ErrorViewModel
+{
+    public string Title { get; init; } = "404";
+
+    public string Subtitle { get; init; } = "Hmmm...";
+
+    public string Body { get; init; } = "It looks like one of the developers fell asleep";
+}

EnvelopeGenerator.GeneratorAPI/Models/Image.cs

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+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;
+}

EnvelopeGenerator.GeneratorAPI/Models/MainViewModel.cs

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+namespace EnvelopeGenerator.GeneratorAPI.Models;
+
+public class MainViewModel
+{
+    public string? Title { get; init; }
+}

EnvelopeGenerator.GeneratorAPI/Models/TFARegParams.cs

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+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);
+}