Story-3159: Curation editor Save/Cancel commit flow + pills-from-preview

The matter-type participant-label curation editor moves from a write-on-every-click model to the app-standard Save / Cancel commit flow. Edits now stage locally — the catalog table does not change until Save — and one atomic reconcile call persists the whole ordered set. The table's Participant-labels column renders from a new list preview, so there is no per-row fetch and no count→name flash.

What a firm admin sees

Open a matter type's Participant labels editor. Attaching an existing label, minting a new one, or removing one now changes only the editor's staged list; the row behind the sheet is untouched. The footer carries Cancel and Save — Save is disabled until there is something to commit. Closing with unsaved changes (Cancel, Escape, or ✕) raises a discard confirm; Save commits everything at once and the table updates instantly.

Staged edits, one commit

One endpoint, one snapshot

The retired per-op routes (attach, create-and-attach, detach, order) are gone. The editor sends the desired set to the batch reconcile endpoint — existing labels by participantLabelId, staged mints by name (the backend dedups, restores, or mints) — and the backend detaches, attaches, and re-orders to match in a single call. On Save-click the editor takes one optimistic snapshot of both the label-list cache and the catalogue row (preview + count), reconciles both from the endpoint's authoritative returned list, and rolls both back with a toast if the write is rejected. The old 204-detach contract mismatch disappears with the endpoint.

Pills straight from the list preview

The matter-types list item now carries attachedParticipantLabelPreview — the first few attached labels in display order — alongside the total attachedParticipantLabels count. The catalog cell renders the preview names as pills with a +N overflow computed from the count, and no longer issues a per-row read. The prior model showed a count pill that flickered into names once the lazy fetch landed; that flash is gone.

Contract and automated proof

The vendored OpenAPI mirror and generated client are synced from the Story-3158 producer contract. The reconcile BFF route validates the request body and asserts the returned MatterTypeParticipantLabelListResponse at the trust boundary (HR-21). Hook tests exercise the flow end to end: staging leaves the table unchanged, Save patches the table optimistically and reconciles from the returned list, a failed Save rolls the table back with a toast, and Cancel discards. Storybook covers the staged / dirty / saving / read-only / empty / loading / error editor states and the pills-from-preview +N overflow.