fix: explicitly specify qmenu arrow width

[iamllama]

Linked issue (required)

Closes: #5420

Summary / motivation (required)

I looked at 6.11.1's changelog and think its due to QTBUG-144914's fix. Providing a size (width in this case) gets us back to expected behaviour

It's also possible to increase QMenu::item's right padding instead to account for the arrow, but then it isn't vertically aligned anymore. Not sure if this is intended or a bug (not fixed by 6.11.2)

Steps to reproduce (required, use N/A if not applicable)

See linked issue

How to test (required)

See linked issue

Checklist (minimum)

• I ran ./ninja check or an equivalent relevant check locally.
• I added or updated tests when the change is non-trivial or behavior changed.

Scope

• This PR is focused on one change (no unrelated edits).