Shared form for writeThrough and owned modes. Shows the ADR-002
disclaimer only in writeThrough mode (no client-side validation
stricter than legacy provides). Maps 400 ErrorsResponse to per-field
inline errors, 422/409 to a banner with the server's own text -
never a client-side switch over invariant names. Embedded in
case-detail with reload() on save so the actions block refreshes.