Added checking for updates

Services/IDownloadService.cs

@@ -0,0 +1,37 @@
+namespace MossyUpdater.Services
+{
+    public interface IDownloadService
+    {
+        Task<DownloadResult> DownloadFileAsync(string url, string destinationPath, IProgress<DownloadProgress>? progress = null, CancellationToken cancellationToken = default);
+        Task<FileUpdateInfo> CheckForUpdatesAsync(string url, string localFilePath, CancellationToken cancellationToken = default);
+        void UnblockFile(string filePath);
+    }
+
+    public class DownloadResult
+    {
+        public bool Success { get; set; }
+        public string? ErrorMessage { get; set; }
+        public long BytesDownloaded { get; set; }
+        public TimeSpan Duration { get; set; }
+    }
+
+    public class DownloadProgress
+    {
+        public long BytesReceived { get; set; }
+        public long? TotalBytes { get; set; }
+        public double ProgressPercentage => TotalBytes.HasValue && TotalBytes > 0 
+            ? (double)BytesReceived / TotalBytes.Value * 100 
+            : 0;
+        public string Status { get; set; } = string.Empty;
+    }
+
+    public class FileUpdateInfo
+    {
+        public bool UpdateAvailable { get; set; }
+        public DateTime? RemoteLastModified { get; set; }
+        public DateTime? LocalLastModified { get; set; }
+        public long? RemoteSize { get; set; }
+        public long? LocalSize { get; set; }
+        public string? ErrorMessage { get; set; }
+    }
+}