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/server/h_status.go

@@ -1,7 +1,10 @@
 package server
 
 import (
+	"context"
+	"fmt"
 	"net/http"
+	"os"
 	"sort"
 	"strings"
 	"sync"
@@ -44,10 +47,43 @@ func (s *Server) status(w http.ResponseWriter, r *http.Request, p *auth.Principa
 	if data, err := s.fw.ListRuleset(r.Context()); err == nil {
 		out.OtherTables = otherTables(data)
 	}
+	out.Problems = append(out.Problems, s.hostProblems(r.Context(), out.OtherTables)...)
 	writeJSON(w, http.StatusOK, out)
 	return nil
 }
 
+const importedTableKey = "imported_table"
+
+// nftablesUnitLinks are where an enabled nftables.service shows (Debian
+// enables it for sysinit.target).
+var nftablesUnitLinks = []string{
+	"/etc/systemd/system/sysinit.target.wants/nftables.service",
+	"/etc/systemd/system/multi-user.target.wants/nftables.service",
+}
+
+// hostProblems are warnings about the machine rather than the
+// configuration: another ruleset that still loads at boot, or the table
+// an import came from, still in force beside Housecarl's.
+func (s *Server) hostProblems(ctx context.Context, others []string) []validate.Problem {
+	var out []validate.Problem
+	for _, link := range s.unitLinks {
+		if _, err := os.Lstat(link); err == nil {
+			out = append(out, validate.Problem{Severity: validate.Warning, Code: "nftables_service", Object: "nftables.service",
+				Message: "nftables.service is enabled: at boot it loads /etc/nftables.conf, and stopping or restarting it flushes every table, Housecarl's included; disable it (systemctl disable nftables, without --now)"})
+			break
+		}
+	}
+	if t, err := s.store.GetMeta(ctx, importedTableKey); err == nil && t != "" {
+		for _, o := range others {
+			if o == t {
+				out = append(out, validate.Problem{Severity: validate.Warning, Code: "takeover_pending", Object: "table " + t,
+					Message: fmt.Sprintf("%s, the table you imported, still applies beside Housecarl's: a packet must get past both; once Housecarl's rules are right, delete it with 'hc takeover %q'", t, t)})
+			}
+		}
+	}
+	return out
+}
+
 // hostInterfaces lists the machine's interfaces with their zones.
 func (s *Server) hostInterfaces(st *model.State) []api.HostInterface {
 	out := []api.HostInterface{}