Add Stop Podman to Settings, alongside Start/Restart

Rounds out the Podman Service card to the full start/stop/restart trio.
Stop and Restart both get a confirm() first — both actually stop every
running container (with its own configured grace period) before touching
the API service itself, not just the service process; worth surfacing
that explicitly rather than letting it be a surprise.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

webui/plugins/podman/ajax/settings.php

@@ -18,6 +18,7 @@
  *   autostart_save  POST {"names": ["postgres", "nextcloud", ...]}
  *   service_status  GET  -> {"running": bool, "output": "..."}
  *   service_start   POST -> {"running": bool, "output": "..."}
+ *   service_stop    POST -> {"running": bool, "output": "..."}
  *   service_restart POST -> {"running": bool, "output": "..."}
  */
 
@@ -58,6 +59,10 @@ switch ($action) {
         podman_json_response(rc_podman('start', 120));
         break;
 
+    case 'service_stop':
+        podman_json_response(rc_podman('stop', 120));
+        break;
+
     case 'service_restart':
         podman_json_response(rc_podman('restart', 120));
         break;