Add a firewall page to the web UI

Allow rules between zones (LANs and tunnels) are edited on their own page:
from, to, protocol, ports, destination. Port forwards and rules with more
options are listed and kept as they are.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AeFqV9dZrK6nipHkPfyM4F

pkgs/router-webui/templates/layout.html

@@ -17,6 +17,7 @@
     <a href="/dhcp">DHCP</a>
     <a href="/dns">DNS</a>
     <a href="/wireguard">WireGuard</a>
+    <a href="/firewall">Firewall</a>
     <a href="/update">Update</a>
     <a href="/advanced">Advanced</a>
     <form method="post" action="/logout"><input type="hidden" name="csrf" value="{{.CSRF}}"><button class="link">Log out</button></form>