fix(autoping): align warning command with on|off

The warning toggle uses _cmd_set_boolean_preference (accepts on|off) and the
subcommand is "warning", but the help syntax, console status lines and warning
text advertised "warning enable|disable" and "/autoping warn", none of which
work.

- cmd_defs: document "/autoping warning on|off"
- console: show "(/autoping warning)" in the status lines
- iq: point the warning text at "/autoping warning off"
- cmd_ac: register "warning" and complete on|off via a dedicated
  _autoping_autocomplete (moved off the generic single-level table)

src/xmpp/iq.c

@@ -2449,7 +2449,7 @@ _disco_autoping_warning_message(GHashTable* features)
         cons_show("This server supports XEP-0199: XMPP Ping (better keepalive detection),\n"
                   "but autoping feature is currently disabled in settings.\n"
                   "Consider enabling it (e.g., `/autoping set 30`) for improved connection stability.\n"
-                  "Use `/autoping warn disable` to disable this message.");
+                  "Use `/autoping warning off` to disable this message.");
     }
     return 0;
 }