Closes https://github.com/payloadcms/payload/issues/7867 Problem: currently, setting an ```ts admin: { width: '30%' } ``` does not work for fields inside a row or similar (group, array etc.) Solution: when we render the field, we set a CSS variable `--field-width` with the value of `admin.width`. This allows us to calculate the correct width for a field in CSS by doing `flex: 0 1 var(--field-width);` It also allows us to properly handle `gap` with `flex-wrap: wrap;` Notes: added playwright tests to ensure widths are correctly rendered 