Fix: Make Scrollbar Thickness Consistent Across DPI

[askadityapandey]

This PR updates scrollbar painting to use a fixed device-pixel
thickness for the gutter and thumb so the UI looks consistent
across different zoom levels and DPI scales.

Key changes

• Introduces fixed device-pixel thickness for scrollbar chrome.
• Converts that thickness to CSS pixels using device_pixels_per_css_pixel().
• Adjusts gutter/thumb rects so right/bottom edges remain anchored.
• Only affects PaintPhase::Overlay and elements whose scrollbar is not none.

Notes

• Visual-only sizing change; scrolling behaviour is unchanged.
• If wanted, this can be exposed via CSS scrollbar-width later.

Fixes #6920

[ladybird-bot]

Hello!

One or more of the commit messages in this PR do not match the Ladybird code submission policy, please check the lint_commits CI job for more details on which commits were flagged and why.
Please do not close this PR and open another, instead modify your commit message(s) with git commit --amend and force push those changes to update this PR.

[konradekk]

(note: there's scrollbar-width checked above, it can contain thin value. looks like a short stretch to add that here, while you are at this.)