style: Update installation security notice header emoji for consistency

Author: stephanebouget

README.md

@@ -6,7 +6,7 @@ A modern desktop application that monitors security vulnerabilities across your
 
 <img width="829" height="102" alt="image" src="https://github.com/user-attachments/assets/69e1730a-720b-4647-9025-6c776309b75e" />
 
-## 🔒 Installation Security Notice
+## 🛡️ Installation Security Notice
 
 When installing this application, you may encounter security warnings from your operating system.