PR 19078: the Import YAML dialog, master vs Steven's PR vs ours

Where it stands: items 1–7 done on our branch, nothing pushed

Our branch is Steven's three commits rebased onto master (55438bb), plus three of ours. It only exists locally and in the preview. Item 8 (the required modal prop) is left for later, as agreed.

1. RcModal's own layout: no wrapper divs, its 24px section gap and 16px action gap. Import is in #primary-action, so RcModal supplies the tertiary Cancel (rc-modal-cancel). The page object follows, which fixes the a11y job.
2. The CSS RcModal made unnecessary is gone (.medium, button padding, .mb-24, the 100% width, text-right).
3. Import is an RcButton with the upload icon, driven by primaryAction, with aria-busy while importing.
4. Cancel, Esc and a click outside do nothing while an import is running.
5. The select says "Default Namespace" again (existing string) and is named by that label. That needed a two-line fix in LabeledSelect, which was dropping aria-label/aria-labelledby. It's its own commit, and it also fixes the PodSecurityAdmission and chart callers that already pass them.
6. The resourceName prop and its strings are gone; the banner says "resources".
7. The unit tests are rewritten on the real RcModal (15 tests, no any). inheritAttrs: false keeps the host's resources/cluster off the dialog element.