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/zh-TW.json

@@ -1410,6 +1410,7 @@
       "smtpTestSuccess": "測試郵件已成功傳送",
       "smtpHostNotConfigured": "尚未設定 SMTP 主機",
       "smtpNoRecipients": "尚未設定收件人",
+      "smtpFromNotConfigured": "未設定 SMTP 寄件人地址",
       "eventLoginAttempt": "登入嘗試",
       "telegramTokenConfigured": "已設定；留空以保留目前的權杖。",
       "telegramTokenPlaceholder": "已設定 - 輸入新權杖以取代",