fix: issue with CSS styles when in-between whole pixel values for viewport width

[davwheat]

Fixes #2915

Changes proposed in this pull request:

Fixes issue where custom UI scaling, combined with specific viewport widths, would result in a broken UI.

This will not fix extensions which use explicit media queries rather than our built-in variable-based ones.

Necessity

• Has the problem that is being solved here been clearly explained?
• If applicable, have various options for solving this problem been considered?
• For core PRs, does this need to be in core, or could it be in an extension?
• Are we willing to maintain this for years / potentially forever?

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Backend changes: tests are green (run composer test).
• Core developer confirmed locally this works as intended.
• Tests have been added, or are not appropriate here.

[askvortsov1]

Can we have a comment about this? Otherwise it's quite confusing why 0.02 exactly.

[davwheat]

Done.