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/en-US.json

@@ -1529,6 +1529,7 @@
       "smtpTestSuccess": "Test email sent successfully",
       "smtpHostNotConfigured": "SMTP host not configured",
       "smtpNoRecipients": "No recipients configured",
+      "smtpFromNotConfigured": "SMTP sender address not configured",
       "eventLoginAttempt": "Login attempt",
       "telegramTokenConfigured": "Configured; leave blank to keep current token.",
       "telegramTokenPlaceholder": "Configured - enter a new token to replace",