status: cancel-all reverts pending changes to last applied config

- lib.common.revert_to_applied(): restore a config file from its
  _last_applied_config snapshot (stamped hash); no baseline -> skip with
  reason, file untouched
- firewall config_apply now stamps the applied baseline like the other
  subsystems; GET /firewall/config and the state collector strip the
  internal _last_applied_* keys
- POST /status/cancel-all + /api/status/cancel-all: revert pending
  subsystems, {cancelled, skipped, errors}, partial-failure safe
- dashboard: "Cancel All Changes" button with confirm modal
  (CancelConfirm, reuses the pending-changes modal rows); the pending
  changes card is hidden entirely when nothing is pending
- tests: revert_to_applied, status_cancel_all, firewall stamping/meta
  stripping, /api/status/cancel-all route, node tests for CancelConfirm;
  firewall _config_apply tests no longer write the real repo config
- docs: api.md, state-model.md, config.md, hoover.md

daemon/iface.py

@@ -209,6 +209,7 @@ GET_WS: Endpoint = _ep("GET", "/ws")
 POST_BATCH: Endpoint = _ep("POST", "/batch")
 GET_STATUS_PENDING: Endpoint = _ep("GET", "/status/pending")
 POST_STATUS_APPLY_ALL: Endpoint = _ep("POST", "/status/apply-all")
+POST_STATUS_CANCEL_ALL: Endpoint = _ep("POST", "/status/cancel-all")
 GET_SYSTEM_METRICS: Endpoint = _ep("GET", "/system/metrics")
 
 # Collect all endpoint module-level constants for __all__ verification