refactor(configmanager): split into Core, CLI, and UI projects

Extract shared models, services, and application logic into
JdeScoping.ConfigManager.Core library. Add JdeScoping.ConfigManager.Cli
console app with validate, test-connection, and secret commands using
System.CommandLine. UI project now references Core for platform-agnostic
functionality while retaining Avalonia-specific dialog and clipboard services.

NEW/src/Utils/JdeScoping.ConfigManager.Core/Services/IBackupService.cs

@@ -0,0 +1,39 @@
+namespace JdeScoping.ConfigManager.Core.Services;
+
+/// <summary>
+/// Service for managing configuration file backups.
+/// </summary>
+public interface IBackupService
+{
+    /// <summary>
+    /// Creates a backup copy of the specified file with a timestamp suffix.
+    /// </summary>
+    /// <param name="filePath">The path of the file to backup.</param>
+    /// <param name="ct">Cancellation token for the operation.</param>
+    /// <returns>The path of the created backup file.</returns>
+    Task<string> CreateBackupAsync(string filePath, CancellationToken ct = default);
+
+    /// <summary>
+    /// Retrieves a list of existing backups for the specified file, sorted by timestamp descending.
+    /// </summary>
+    /// <param name="filePath">The original file path to find backups for.</param>
+    /// <param name="ct">Cancellation token for the operation.</param>
+    /// <returns>A read-only list of backup information sorted by timestamp.</returns>
+    Task<IReadOnlyList<BackupInfo>> GetBackupsAsync(string filePath, CancellationToken ct = default);
+
+    /// <summary>
+    /// Restores a backup file by copying it to the target location.
+    /// </summary>
+    /// <param name="backupPath">The path of the backup file to restore from.</param>
+    /// <param name="targetPath">The target path where the backup should be restored.</param>
+    /// <param name="ct">Cancellation token for the operation.</param>
+    Task RestoreBackupAsync(string backupPath, string targetPath, CancellationToken ct = default);
+
+    /// <summary>
+    /// Removes old backup files, keeping only the most recent backups.
+    /// </summary>
+    /// <param name="filePath">The original file path to find backups for.</param>
+    /// <param name="keepCount">The number of most recent backups to retain (default: 10).</param>
+    /// <param name="ct">Cancellation token for the operation.</param>
+    Task CleanupOldBackupsAsync(string filePath, int keepCount = 10, CancellationToken ct = default);
+}