fix(trace viewer): Screenshot click target placement

[Skn0tt]

This closes the last element of #32847, so it reverts the changes where we hide the toggle.

Misplacement before:

Screen.Recording.2024-10-15.at.11.38.02.mov

Correct placement after:

Screen.Recording.2024-10-15.at.11.36.59.mov

The bug was two-fold.

1. Because it's rendered as position: fixed, we need to take the <BrowserFrame />'s height into account.
2. Our centering with margin: -10px 0 0 -10px was relying on a 20px width that wasn't perfect. transform works better, learned that through https://css-tricks.com/quick-css-trick-how-to-center-an-object-exactly-in-the-center/.

Closes #32847

[github-actions]

Test results for "tests 1"

2 fatal errors, not part of any test

1 flaky

⚠️ [webkit-library] › library/screenshot.spec.ts:204:14 › element screenshot › element screenshot should work with a mobile viewport @webkit-ubuntu-22.04-node18

35966 passed, 612 skipped, 1 did not run
✔️✔️✔️

Merge workflow run.

[Skn0tt]

closing in favour of #33119