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/ar-EG.json

@@ -1410,6 +1410,7 @@
       "smtpTestSuccess": "تم إرسال البريد التجريبي بنجاح",
       "smtpHostNotConfigured": "خادم SMTP غير مهيأ",
       "smtpNoRecipients": "لا يوجد مستلمون مهيؤون",
+      "smtpFromNotConfigured": "عنوان مرسل SMTP غير مُهيأ",
       "eventLoginAttempt": "محاولة تسجيل دخول",
       "telegramTokenConfigured": "مهيأ؛ اتركه فارغاً للاحتفاظ بالتوكن الحالي.",
       "telegramTokenPlaceholder": "مهيأ — أدخل توكن جديد لاستبداله",