Extract a common, reusable loading spinner (nx2)

[usman-khalid]

Follow-up from review feedback on #592

nx2 has multiple near-identical rotating border-spinners, each defining its own @keyframes:

• nx2/blocks/ew-actions/ew-actions.css — .preview-dropdown-spinner + @keyframes preview-dropdown-spin (added in feat(ew): show loading spinner on preview & publish #592)
• nx2/styles/form.css — .da-checkbox.loading + @keyframes da-spin
• nx2/public/plugins/quick-edit/quick-edit.css — picture.image-uploading::after + @keyframes spin

All three are the same pattern (border + border-top-color transparent + border-radius: 50% + rotate(360deg)), just with different sizes/colors and duplicated keyframes.

Should consolidate and re-use across these UI surfaces.