dhcp: auto-populate gateway from interface IP for DHCP ranges

Add get_interface_ip() helper to resolve an interface's IPv4 address
via 'ip -o addr show'.  Use it to back-propagate gateway into DHCP
ranges so clients receive their default route.

- set_dhcp_range() resolves gateway: explicit > existing range > iface IP
- DnsToFirewallSync and FirewallToDhcpSync sync ensure gateways are set
- Remove automatic masquerade toggle from DnsToFirewallSync
- Fix dnsmasq lease file path to /var/lib/misc/dnsmasq.leases
- Rename lease state field expires_at -> expires (ISO string)
- Add 'ip -o addr show' to sudo whitelist

webui/static/pages/zones.js

@@ -63,7 +63,7 @@ export default definePage({
                         onClick=${() => MultiSelectModal({
                             title: 'Interfaces: ' + name,
                             url: '/api/firewall/zones/' + enc(name) + '/interfaces',
-                            options: state.firewall.data?.interfaces || [],
+                            options: (state.firewall.data?.interfaces || []).map(i => i.name),
                             selected: ifacesArr,
                             fieldKey: 'interfaces',
                             successMsg: 'Interfaces updated',