Final polish: add constant for buffer size, use string interpolation, fix compatibility

Co-authored-by: BornToBeRoot <16019165+BornToBeRoot@users.noreply.github.com>

Author: Copilot

Source/NETworkManager.Profiles/ProfileManager.cs

@@ -57,6 +57,11 @@ public static class ProfileManager
         Converters = { new JsonStringEnumConverter() }
     };
 
+    /// <summary>
+    ///     Maximum number of bytes to check for XML content detection.
+    /// </summary>
+    private const int XmlDetectionBufferSize = 200;
+
     /// <summary>
     ///     ObservableCollection of all profile files.
     /// </summary>
@@ -167,9 +172,9 @@ private static string GetJsonProfilePath(string originalPath, bool encrypted)
         var basePath = Path.ChangeExtension(originalPath, null);
 
         if (encrypted)
-            return basePath + ProfileFileExtension + ProfileFileExtensionEncrypted;
+            return $"{basePath}{ProfileFileExtension}{ProfileFileExtensionEncrypted}";
 
-        return basePath + ProfileFileExtension;
+        return $"{basePath}{ProfileFileExtension}";
     }
 
     /// <summary>
@@ -521,7 +526,7 @@ private static void Load(ProfileFileInfo profileFileInfo)
                     var originalPath = profileFileInfo.Path;
 
                     // Migrate to JSON format by saving the file
-                    var newPath = Path.ChangeExtension(profileFileInfo.Path, ProfileFileExtension + ProfileFileExtensionEncrypted);
+                    var newPath = GetJsonProfilePath(profileFileInfo.Path, true);
                     var newProfileFileInfo = new ProfileFileInfo(profileFileInfo.Name, newPath, true)
                     {
                         Password = profileFileInfo.Password,
@@ -638,8 +643,8 @@ private static bool IsXmlContent(byte[] data)
 
         try
         {
-            // Only check the first 200 bytes for performance
-            var bytesToCheck = Math.Min(200, data.Length);
+            // Only check the first few bytes for performance
+            var bytesToCheck = Math.Min(XmlDetectionBufferSize, data.Length);
             var text = Encoding.UTF8.GetString(data, 0, bytesToCheck).TrimStart();
             // Check for XML declaration or root element that matches profile structure
             return text.StartsWith("<?xml") || text.StartsWith("<ArrayOfGroupInfoSerializable");
@@ -896,7 +901,7 @@ private static List<GroupInfo> DeserializeGroup(List<GroupInfoSerializable> grou
             throw new ArgumentNullException(nameof(groupsSerializable));
 
         return (from groupSerializable in groupsSerializable
-                let profiles = (groupSerializable.Profiles ?? []).Select(profileSerializable => new ProfileInfo(profileSerializable)
+                let profiles = (groupSerializable.Profiles ?? new List<ProfileInfoSerializable>()).Select(profileSerializable => new ProfileInfo(profileSerializable)
                 {
                     // Migrate old tags to new tags list
                     // if TagsList is null or empty and Tags is not null or empty, split Tags by ';' and create a new ObservableSetCollection