fix(07): SMTP test warns when connection passes but no auth configured

Server reachable without credentials (port 25 open relay) returns
{ success: true, warning: 'no_auth' } instead of green success.
Frontend shows red warning: server reachable but emails will fail.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/web/src/lib/settings-api.ts

@@ -77,7 +77,7 @@ export async function saveSmtp(payload: SaveSmtpPayload): Promise<SmtpConfig> {
  */
 export async function testSmtp(
   payload: SaveSmtpPayload,
-): Promise<{ success: boolean }> {
+): Promise<{ success: boolean; warning?: string }> {
   const res = await fetch(`${API_URL}/settings/smtp/test`, {
     method: 'POST',
     headers: { 'Content-Type': 'application/json' },