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

docs/config.md

@@ -538,6 +538,8 @@ Both `/api/firewall/zones/<name>/services` and `/api/firewall/config/apply` reco
 
 **Management-lockout guard.** The firewalld *default zone* is the catch-all for interfaces with no explicit assignment (typically the WAN), and it carries the management plane (nginx https) plus remote recovery (ssh). Changing the default zone's service set so that **neither `https` nor `ssh`** remains raises `409 Conflict` — from `POST /firewall/zones/<name>/services` and `POST /firewall/config/apply` — before any mutation runs. Send `"force": true` in the request body to override (the UI shows a confirm dialog with this effect on the Zones page). If the default zone cannot be determined, the guard fails closed.
 
+**Applied baseline.** Like the other config-backed subsystems, a successful apply records `_last_applied_hash` and `_last_applied_config` (the meta-stripped config snapshot) inside `config.json`. They are internal bookkeeping — ignored by all parsing, hashing, and UI surfaces — and let the aggregate cancel action (`POST /api/status/cancel-all`) revert this file to the last applied state. Configs that have never been applied have no baseline and are skipped by cancel.
+
 ## Networkd (IP Configuration)
 
 **File**: `config/network/config.json`