fix(settings): show the SMTP failure reason instead of a raw i18n key

classifySMTPError returned keys already carrying "pages.settings.", while the
four keys TestConnection returns directly do not, and the alert renders every
Message under that one prefix. Any classified failure therefore looked up
pages.settings.pages.settings.smtpErrorAuth, which does not exist, so the panel
printed the key instead of "Authentication failed — check username and
password". The unknown case was worse: it appended the raw error to the key, and
its own text interpolated {{ .Error }}, Go template syntax the frontend's
i18next never fills.

Return the keys unprefixed like the rest, and point the unknown case at the
panel log, which already carries the underlying error.

internal/web/translation/es-ES.json

@@ -1394,7 +1394,7 @@
       "smtpErrorTimeout": "Tiempo de conexión agotado: servidor inaccesible",
       "smtpErrorRelay": "El servidor rechaza el envío desde esta dirección",
       "smtpErrorEof": "Conexión cerrada por el servidor",
-      "smtpErrorUnknown": "Error de SMTP: {{ .Error }}",
+      "smtpErrorUnknown": "Error de SMTP — consulta el registro del panel",
       "eventMemoryHigh": "Uso de memoria alto (%)",
       "remarkTemplate": "Plantilla de notas",
       "remarkTemplateDesc": "Cuando se define, esto reemplaza el modelo de notas para cada enlace de suscripción — escribe tu propio formato con los tokens de variable (usa el botón para insertarlos). Déjalo vacío para usar el modelo anterior.",