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.

Start by reproducing the save-and-cancel flow in the desktop Graphite app on Windows, then trace how cancelling Save affects the document's modified state and close prompt. Done means cancelling either normal saving or Save As preserves the unsaved indicator and closing the document shows the save warning.

Written by the indexing model from the issue text.