feat(onboarding): mirror offer uses the create wizard's region selection

The offer hardcoded aws-us-east-2 while entire repo mirror create offers the catalog: bare wizard = region multi-select, one-shot = cluster picker. Setup now resolves placements with the wizard's own machinery — the region multi-select pre-checked to the caller's home-jurisdiction default (one Enter for the common case, several regions selectable — data residency is the user's call in setup exactly as in the command), pickRegions + createMirrors shared verbatim. One catalog region skips the question; non-interactive keeps the fixed default so scripts stay stable.

finalizeMirrorOffer folds per-region results: any serving placement is a success, suspended-only keeps the honest warning + retry hint, total failure surfaces the cause.