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/import.md

@@ -107,7 +107,10 @@ the old table:
 sudo hc takeover "inet main"
 ```
 
-From now on only Housecarl's rules apply. Over the network, this is on
+From now on only Housecarl's rules apply. Until you do, the dashboard
+and `hc status` warn that the imported table still applies
+(`takeover_pending`); they also warn while `nftables.service` is enabled
+(`nftables_service`). Over the network, this is on
 probation too: unless confirmed, the old table comes back.
 
 ## 4. The boot
@@ -122,6 +125,11 @@ sudo systemctl disable nftables.service
 sudo mv /etc/nftables.conf /etc/nftables.conf.before-housecarl
 ```
 
+Disable it, but **don't stop it** (no `systemctl stop nftables`, no
+`disable --now`): Debian's unit stops by running `nft flush ruleset`,
+which deletes every table, Housecarl's and Docker's included, at once.
+Disabled, it simply doesn't run at the next boot.
+
 `housecarl-ruleset.service` runs after `nftables.service` in any case,
 so a leftover `flush ruleset` can't undo it. Docker, Incus and libvirt
 keep adding their own tables as before.