Fix stored XSS in low security level

[r1kka6]

Summary

• encode guestbook name and message fields before storing them
• preserve the existing low-level database escaping while preventing stored script execution

Verification

• docker run --rm -v "$PWD":/work -w /work php:8.3-cli php -l vulnerabilities/xss_s/source/low.php
• vulnerable baseline renders <script>alert(1)</script> and <script>alert(2)</script> back into the guestbook HTML
• patched runtime renders the same payloads as <script>...</script>