Remember chat provider (#217)

app/MindWork AI Studio/Chat/ChatThread.cs

@@ -15,6 +15,11 @@ public sealed record ChatThread
     /// </summary>
     public Guid WorkspaceId { get; set; }
 
+    /// <summary>
+    /// Specifies the provider selected for the chat thread.
+    /// </summary>
+    public string SelectedProvider { get; set; } = string.Empty;
+
     /// <summary>
     /// The name of the chat thread. Usually generated by an AI model or manually edited by the user.
     /// </summary>