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/model/changeset.go

@@ -35,13 +35,16 @@ const (
 	OpDelete   = "delete"
 	OpMove     = "move"
 	OpSettings = "settings"
+	// OpRevert undoes a history entry: Data is the entry (its changes),
+	// ID its number.
+	OpRevert = "revert"
 )
 
 // Op is one change, as the API received it, so it can be stored and
 // replayed on whatever the configuration is when it is applied.
 type Op struct {
 	Seq  int    `json:"seq"`
-	Op   string `json:"op"` // create, update, delete, move, settings
+	Op   string `json:"op"` // create, update, delete, move, settings, revert
 	Kind string `json:"kind,omitempty"`
 	// Ref names the object (ID, or name) for update, delete and move.
 	Ref string `json:"ref,omitempty"`