Cancelling document saving leads to content/modifications being lost

**Description:**
If cancel saving the document then there will be no warning about unsaved changes when closing the document. This can lead to losing modifications after the document is closed.

**Environment:**
The bug is happening in the desktop version of Graphite app (commit 9f25c50c) on MS Windows 11 Home (v10.0.26200).

**Steps to reproduce:**
1. Open the desktop version of Graphite app.
2. Create the new document.
3. Draw an ellipse in the document.
4. Press Ctrl+S to save the document. (-> the asterisk on the document header disappears already at this point, which is suspicious)
5. In the Save dialog, press the Cancel button.
6. Close the document.

Expected: Graphite app shows the popup with the suggestion to save the document.
Actual: Graphite app silently closes the document, so all the content in the document is lost.

The similar issue is happening when you open document, make changes, start saving it as another document, pressing the Cancel button in the Save dialog and closing the document -> all the changes in the document are lost.

先在 Windows 上的桌面版 Graphite 应用中复现保存和取消流程，然后跟踪取消 Save 如何影响文档的修改状态和关闭提示。完成标准是：取消普通保存或 Save As 后，未保存指示器都能保留，并且关闭文档时会显示保存警告。

由索引模型根据 Issue 内容生成。