feat(nodes): bulk panel self-update with live online indicator

Adds the ability to update node panels to the latest release from the Nodes
page: select online, enabled nodes (checkboxes) and trigger their official
self-updater, or use the per-row Update action. A node whose reported panel
version trails the latest GitHub release is flagged with an 'update available'
tag (compared via lib/panel-version, mirroring the Go isNewerVersion).

Backend: Remote.UpdatePanel calls the node's existing
POST /panel/api/server/updatePanel; NodeService.UpdatePanels fans out over the
selected ids, skipping disabled/offline nodes with a per-node reason; exposed
as POST /panel/api/nodes/updatePanel (documented in endpoints.ts + openapi.json).

The bulk request sends a JSON body, so it sets Content-Type: application/json
explicitly — axios defaults POST to form-urlencoded, which made ShouldBindJSON
fail with 'invalid character i'.

Also reuses the clients-page online cue on the Nodes page: a pulsing green dot
plus green label for an online node. The .online-dot style moved to the shared
styles/utils.css so both pages load it.

Translations for all new node keys added across every language file.

web/translation/es-ES.json

@@ -832,6 +832,12 @@
       "panelVersion": "Versión del panel",
       "actions": "Acciones",
       "probe": "Sondear ahora",
+      "updatePanel": "Actualizar panel",
+      "updateSelected": "Actualizar seleccionados ({count})",
+      "updateAvailable": "Actualización disponible",
+      "upToDate": "Actualizado",
+      "updateConfirmTitle": "¿Actualizar {count} nodo(s) a la última versión?",
+      "updateConfirmContent": "Cada nodo seleccionado descarga la última versión y se reinicia con ella. Solo se actualizan los nodos habilitados y en línea.",
       "testConnection": "Probar conexión",
       "connectionOk": "Conexión correcta ({ms} ms)",
       "connectionFailed": "Conexión fallida",
@@ -853,7 +859,10 @@
         "deleted": "Nodo eliminado",
         "test": "Probar conexión",
         "fillRequired": "El nombre, la dirección, el puerto y el token de API son obligatorios",
-        "probeFailed": "Sondeo fallido"
+        "probeFailed": "Sondeo fallido",
+        "updateStarted": "Actualización del panel iniciada",
+        "updateResult": "Actualización iniciada en {ok} nodo(s), {failed} fallaron",
+        "updateNoneEligible": "Selecciona al menos un nodo en línea y habilitado"
       }
     },
     "settings": {