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/pt-BR.json

@@ -1394,7 +1394,7 @@
       "smtpErrorTimeout": "Tempo de conexão esgotado — host inacessível",
       "smtpErrorRelay": "O servidor rejeita o envio a partir deste endereço",
       "smtpErrorEof": "Conexão encerrada pelo servidor",
-      "smtpErrorUnknown": "Erro de SMTP: {{ .Error }}",
+      "smtpErrorUnknown": "Erro de SMTP — consulte o log do painel",
       "eventMemoryHigh": "Uso de memória alto (%)",
       "remarkTemplate": "Modelo de Observação",
       "remarkTemplateDesc": "Quando definido, isto substitui o modelo de observação de cada link de assinatura — escreva seu próprio formato com os tokens de variáveis (use o botão para inseri-los). Deixe vazio para usar o modelo acima.",