fix(ui): update Spanish translations and add missing gain keys

- Add missing 'albumGain' and 'trackGain' keys (matches recent additions in pt-br, ru)
- Translate 'Playlists' and 'Shared Playlists' to 'Listas de reproducción' / '...compartidas'
- Translate 'OFFLINE' (server down indicator) to 'DESCONECTADO'

Spanish translation now covers 553/553 keys (was 551/553).

Signed-off-by: Daniel Banariba <banaribad@gmail.com>

resources/i18n/es.json

@@ -35,6 +35,8 @@
         "rawTags": "Etiquetas sin procesar",
         "bitDepth": "Profundidad de bits",
         "sampleRate": "Frecuencia de muestreo",
+        "albumGain": "Ganancia de álbum",
+        "trackGain": "Ganancia de pista",
         "missing": "Faltante",
         "libraryName": "Biblioteca",
         "composer": "Compositor",
@@ -625,8 +627,8 @@
     },
     "albumList": "Álbumes",
     "about": "Acerca de",
-    "playlists": "Playlists",
-    "sharedPlaylists": "Playlists Compartidas",
+    "playlists": "Listas de reproducción",
+    "sharedPlaylists": "Listas de reproducción compartidas",
     "librarySelector": {
       "allLibraries": "Todas las bibliotecas (%{count})",
       "multipleLibraries": "%{selected} de %{total} bibliotecas",
@@ -693,7 +695,7 @@
     "quickScan": "Escaneo rápido",
     "fullScan": "Escaneo completo",
     "serverUptime": "Uptime del servidor",
-    "serverDown": "OFFLINE",
+    "serverDown": "DESCONECTADO",
     "scanType": "Tipo",
     "status": "Error de escaneo",
     "elapsedTime": "Tiempo transcurrido",