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/design.md

@@ -75,8 +75,9 @@ to be reviewed and applied together with others:
 | CLI | Immediate: each command is a change of its own. Stage with `-C ID` or `HOUSECARL_CHANGESET`. | Scripts want a clear result per step. |
 | API | Immediate unless a changeset is named (`?changeset=ID`, `X-Housecarl-Changeset`). | As the CLI. |
 
-Undo, imports and takeovers can't be staged; with a changeset named, they
-are refused rather than applied.
+An undo is staged like any edit (in review mode, the web UI's Undo
+collects it for review). Imports and takeovers can't be staged; with a
+changeset named, they are refused rather than applied.
 
 ## Its own table only