Root Tag is added to Extends when deselecting the Root Tag option

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### Search first

- [x] I searched and no similar issues were found

### What Happened?

When configuring the **Extends** property on a tag page, selecting a tag other than **Root Tag** opens a popup that remains open.

If I then click **Root Tag** in the popup to deselect the option, **Root Tag is incorrectly added to the Extends value** instead of simply being removed from the selection.

### Reproduce the Bug

1. Open a tag page.
2. Configure the **Extends** property.
3. Select a tag other than **Root Tag**.
4. While the popup remains open, click **Root Tag** to deselect it.
5. Check the resulting **Extends** value.

### Expected Behavior

Root Tag should not be available as an option in the popup.

### Screenshots

### Files

_No response_

### Browser, Desktop or Mobile Platform Information

windows 11
logseq 2.0.1-alpha+nightly.20260907

### Additional Context

_No response_

### Are you willing to submit a PR? If you know how to fix the bug.

- [ ] I'm willing to submit a PR (Thank you!)

Start by reproducing the issue on a tag page in the Extends property popup, then trace the selection handling for Root Tag when another tag is selected. Done means Root Tag is unavailable in the popup and clicking to deselect it no longer adds Root Tag to the Extends value.

Written by the indexing model from the issue text.