fix(email): report a missing sender address from the SMTP connection test

TestConnection skipped the empty-from guard that Send enforces, so with
no sender and no username configured the test issued the null reverse-path
and could report success against a lenient relay while every real
notification send kept failing with the missing-sender error. Guard the
test path the same way and surface a dedicated translated message.

internal/web/translation/id-ID.json

@@ -1410,6 +1410,7 @@
       "smtpTestSuccess": "Email uji berhasil dikirim",
       "smtpHostNotConfigured": "Host SMTP belum dikonfigurasi",
       "smtpNoRecipients": "Tidak ada penerima yang dikonfigurasi",
+      "smtpFromNotConfigured": "Alamat pengirim SMTP belum dikonfigurasi",
       "eventLoginAttempt": "Percobaan masuk",
       "telegramTokenConfigured": "Terkonfigurasi; kosongkan untuk mempertahankan token saat ini.",
       "telegramTokenPlaceholder": "Terkonfigurasi - masukkan token baru untuk mengganti",