feat: Enable Content Security Policy (CSP)

- Enable $CSPEnabled = true in app/Config/App.php
- CSP directives already configured in ContentSecurityPolicy.php
- Includes support for CSP 3 keywords and directives

Closes #4488

app/Config/App.php

@@ -278,7 +278,7 @@ class App extends BaseConfig
      * @see http://www.html5rocks.com/en/tutorials/security/content-security-policy/
      * @see http://www.w3.org/TR/CSP/
      */
-    public bool $CSPEnabled = false;
+    public bool $CSPEnabled = true;
 
     public function __construct()
     {