feat: Send actual test email instead of just verifying connection

- testConnection now sends a real test email to the configured Gmail address
- User receives actual email confirmation that setup works
- Email contains success message and system info

backend/src/services/mail.service.js

@@ -85,7 +85,18 @@ class MailService {
   async testConnection() {
     try {
       await this.initializeTransporter();
-      return { success: true, message: 'Gmail connection successful' };
+      
+      // Send actual test email
+      const testEmail = await this.sendMail(
+        this.transporter.options.auth.user, // Send to self
+        'Test Mail - Oltalama Paneli',
+        '<h2>✅ Test Başarılı!</h2><p>Gmail ayarlarınız doğru yapılandırılmış ve mail gönderimi çalışıyor.</p><p><strong>Sistem:</strong> Oltalama Test Yönetim Paneli</p>'
+      );
+      
+      return { 
+        success: true, 
+        message: `Test maili başarıyla gönderildi! (${testEmail.messageId})` 
+      };
     } catch (error) {
       return { success: false, error: error.message };
     }