fix(sub): restore the subscription info page for browser visits

Revert 43bc9153 and its follow-up 338822ab. The copy-only notice replaced the
themed sub page for every browser request, so mobile users got a bare "This is
a subscription link" screen instead of their traffic, expiry and links — and it
left serveSubPage plus the custom-theme renderer as dead code.

internal/web/translation/es-ES.json

@@ -2075,8 +2075,5 @@
     "statusFailed": "FALLIDO",
     "statusDown": "CAÍDO",
     "statusUp": "ACTIVO"
-  },
-  "subCopyPageTitle": "Enlace de suscripción",
-  "subCopyPageHeading": "Este es un enlace de suscripción",
-  "subCopyPageInstructions": "No necesita abrirlo en el navegador. Copie la dirección de esta página y péguela en la aplicación."
+  }
 }