Cannot click "Start interview" button when zoomed in (modal unscrollable)

[seallllllllll]

On browser zoom levels >= 150%, the setup modal gets cut off at the bottom. Since the modal lacks a vertical scrollbar, there's no way to scroll down to view the full message or hit the "Start interview" button.

[jserv]

@seallllllllll Open PR #174 carries a separate commit for this. The preflight overlay could not scroll when the card was taller than the window, so the top of the card and the Start interview button were unreachable. Browser zoom shortens the viewport in CSS pixels, which is the same condition. The fix was checked by hand at window heights of 545 px and 1091 px.

Zoom is not what that PR was tested against, so please try the branch at 150% and report on PR #174 whether it clears the symptom. If it does, this issue closes when that PR merges; if it does not, the zoom case is a real gap and this issue should stay open with that result.