Staged undo, built-ins window, machine warnings; raw and dynamic set fixes

- Undo is staged like any edit: in review mode the web UI's Undo, and hc
  history undo with -C or HOUSECARL_CHANGESET, add a revert operation
  that checks, when replayed, that nothing it touches changed since;
  applied, History links the entries. Imports and takeovers still can't
  be staged.
- The rules page sums up a chain's built-ins in one line; a window lists
  them with what they do and their hits, and switches them.
- Warnings about the machine (dashboard, hc status): nftables.service
  enabled (it loads /etc/nftables.conf at boot and flushes every table
  when stopped), and the imported table still in force beside
  Housecarl's. The docs say to disable nftables.service but not stop it.
- A raw rule with other fields set is refused (raw_conflict) instead of
  silently ignoring them; the rule dialog's nftables section hides what
  it replaces, and the importer's raw rules carry only their text.
- A dynamic set with configured entries is refused
  (set_dynamic_entries): they were silently ignored. The set dialog
  shows entries or the entry timeout, as fits.
- The rule dialog's chain choices are just the chains' names.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/api.md

@@ -157,7 +157,10 @@ Changes staged to be reviewed and applied together ([design.md](design.md#review
 Staging (`?changeset=ID` on any change to objects or settings) checks
 the changeset with the new operation, and nftables checks its ruleset;
 an error refuses the operation. Only open changesets take operations, and
-only their owner's (or an administrator's). Undo, `/import/nft` and
+only their owner's (or an administrator's). An undo
+(`POST /history/{id}/revert?changeset=ID`) is staged as one operation,
+which checks when it is replayed that nothing it touches changed since;
+applied, History links it to the entry it undoes. `/import/nft` and
 `/takeover` can't be staged: with a changeset named they answer 400.
 
 Reads of objects, settings and `/ruleset` with `?changeset=ID` show the