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>

internal/client/client.go

@@ -39,9 +39,9 @@ type Client struct {
 }
 
 // Staged reports whether a request is a configuration change that a
-// changeset takes: objects and settings, and the changes that can't be
-// staged (undo, import, takeover), which the server then refuses rather
-// than apply them. Changesets' own calls and dynamic set entries aren't.
+// changeset takes: objects, settings and undo, and the changes that can't
+// be staged (import, takeover), which the server then refuses rather than
+// apply them. Changesets' own calls and dynamic set entries aren't.
 func Staged(method, path string) bool {
 	if method == http.MethodGet || strings.Contains(path, "/elements") {
 		return false