修复CancelButton属性编译错误

将 cancelButton.CancelButton 改为 form.CancelButton
CancelButton是Form的属性，用于设置窗体的取消按钮，不是Button的属性

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Co-Authored-By: Claude <noreply@anthropic.com>

Form1.cs

@@ -347,7 +347,7 @@ namespace SlideCombine
                 cancelButton.ForeColor = Color.White;
                 cancelButton.FlatStyle = FlatStyle.Flat;
                 cancelButton.FlatAppearance.BorderSize = 0;
-                cancelButton.CancelButton = cancelButton;
+                form.CancelButton = cancelButton;
                 form.Controls.Add(cancelButton);
 
                 // 当前路径