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/zh-TW.json

@@ -1394,7 +1394,7 @@
       "smtpErrorTimeout": "連線逾時 — 無法連線至主機",
       "smtpErrorRelay": "伺服器拒絕從此地址傳送",
       "smtpErrorEof": "連線已被伺服器關閉",
-      "smtpErrorUnknown": "SMTP 錯誤：{{ .Error }}",
+      "smtpErrorUnknown": "SMTP 錯誤 — 詳情見面板日誌",
       "eventMemoryHigh": "記憶體使用率高 (%)",
       "remarkTemplate": "備註範本",
       "remarkTemplateDesc": "設定後，這將取代每個訂閱連結的備註模型——使用變數標記撰寫您自己的格式（使用按鈕來插入）。留空則使用上方的模型。",