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-CN.json

@@ -1410,6 +1410,7 @@
       "smtpTestSuccess": "测试邮件发送成功",
       "smtpHostNotConfigured": "尚未配置 SMTP 主机",
       "smtpNoRecipients": "尚未配置收件人",
+      "smtpFromNotConfigured": "未配置 SMTP 发件人地址",
       "eventLoginAttempt": "登录尝试",
       "telegramTokenConfigured": "已配置；留空则保留当前令牌。",
       "telegramTokenPlaceholder": "已配置——输入新令牌以替换",