Add a clickable exit button for menu in mobile UI

[lpmi-13]

Describe Your Changes

In a mobile screen, there's no way to easily exit the Graph Settings menu, which takes up the whole screen. Here's what it looks like on the current master branch:

This is likely because in the full desktop view, it doesn't take up the entire screen and so it doesn't need a way to be closed. Clicking anywhere else on the page removes focus and it disappears. In order to make the smallest possible change to address this, I've added a small close button, which looks like this in the current commit:

If, however, it would be preferable to change this approach so that the menu doesn't take up the entire screen and clicking outside the element removes focus, making it disappear, I can do that instead.

Checklist

The following checks are mandatory:

• My change adheres to VictoriaMetrics contributing guidelines.
• My change adheres to VictoriaMetrics development goals.

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Summary by cubic

Fixes the mobile Graph Settings menu by showing it as a modal with a close button, so users can exit it. Desktop behavior is unchanged.

^{Written for commit c404931. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 3 files