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

lib/state.py

@@ -523,7 +523,7 @@ def _collect_dnsmasq() -> dict[str, Any]:
     CONFIG_DIR = PROJECT_DIR / "config" / "dnsmasq"
     CONFIG_PATH = CONFIG_DIR / "config.json"
     DNSMASQ_CONF = "/etc/dnsmasq.d/vacuum-wall.conf"
-    LEASE_FILE = "/var/lib/dnsmasq/dnsmasq.leases"
+    LEASE_FILE = "/var/lib/misc/dnsmasq.leases"
 
     DEFAULT_CFG: dict[str, Any] = {
         "dhcp": {"ranges": [], "static_leases": []},
@@ -575,7 +575,7 @@ def _collect_dnsmasq() -> dict[str, Any]:
                 ts = None
             leases.append(
                 {
-                    "expires_at": ts,
+                    "expires": ts.isoformat() if ts else "",
                     "mac": parts[1],
                     "ip": parts[2],
                     "hostname": parts[3] if len(parts) > 3 else "",