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/id-ID.json

@@ -1394,7 +1394,7 @@
       "smtpErrorTimeout": "Koneksi waktu habis — host tidak dapat dijangkau",
       "smtpErrorRelay": "Server menolak pengiriman dari alamat ini",
       "smtpErrorEof": "Koneksi ditutup oleh server",
-      "smtpErrorUnknown": "Kesalahan SMTP: {{ .Error }}",
+      "smtpErrorUnknown": "Kesalahan SMTP — lihat log panel",
       "eventMemoryHigh": "Penggunaan memori tinggi (%)",
       "remarkTemplate": "Templat Catatan",
       "remarkTemplateDesc": "Jika diatur, ini menggantikan model catatan untuk setiap tautan langganan — tulis format Anda sendiri dengan token variabel (gunakan tombol untuk menyisipkannya). Biarkan kosong untuk memakai model di atas.",