- Add SecurityHeadersMiddleware that sets Content-Security-Policy,
  X-Content-Type-Options, X-Frame-Options, X-Permitted-Cross-Domain-Policies,
  X-DNS-Prefetch-Control, Referrer-Policy, and X-Robots-Tag on all responses
- Register SecurityHeadersMiddleware in index.php
- Add click-handlers.js for CSP-compliant event handling (data-confirm,
  data-stop-event-propagation)
- Update toggle-dark-mode.js to attach click handler via addEventListener
- Remove inline onclick from theme toggle button in layout.twig
- Replace all inline onclick with data-confirm in containers.twig,
  community-containers.twig, and optional-containers.twig

Co-authored-by: szaimen <42591237+szaimen@users.noreply.github.com>
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