feat: Add main application views and structure

- Implemented CreateInstanceView for creating new instances.
- Added HostsView for managing SSH hosts with CRUD operations.
- Created InstancesView for displaying and managing instances.
- Developed LogsView for viewing operation logs.
- Introduced SecretsView for managing secrets associated with hosts.
- Established SettingsView for configuring application settings.
- Created MainWindow as the main application window with navigation.
- Added app manifest and configuration files for logging and settings.

OTSSignsOrchestrator.Core/Models/Entities/AppSetting.cs

@@ -0,0 +1,23 @@
+using System.ComponentModel.DataAnnotations;
+
+namespace OTSSignsOrchestrator.Core.Models.Entities;
+
+/// <summary>
+/// Key-value application setting persisted in the local database.
+/// Sensitive values are encrypted at rest via DataProtection.
+/// </summary>
+public class AppSetting
+{
+    [Key, MaxLength(200)]
+    public string Key { get; set; } = string.Empty;
+
+    [MaxLength(4000)]
+    public string? Value { get; set; }
+
+    [Required, MaxLength(50)]
+    public string Category { get; set; } = string.Empty;
+
+    public bool IsSensitive { get; set; }
+
+    public DateTime UpdatedAt { get; set; } = DateTime.UtcNow;
+}