Fix format bar position

[florianduros]

Checklist

• Tests written for new code (and old code if feasible).
• New or updated public/exported symbols have accurate TSDoc documentation.
• Linter and other CI checks pass.
• I have licensed the changes to Element by completing the Contributor License Agreement (CLA)

Closes #28532

• When we want to display the format bar, the component doesn't have yet its height. Since the height is hardcoded in CSS, we can use also an hardcoded value for the computation.
• Added 1 px more than the initial computation which didn't include the border size, it allows the selection to not overflow over the formatting bar.

[dbkr]

Thanks!