feat: add gateway setup wizard and fix test suite

New zt-gateway-setup script with subcommands for configuring this
router as a ZeroTier exit gateway: bridge, routing (tables 100/101),
DHCP, hotplug, and persistence. Exposes setup ubus method via backend
with ACL and LuCI panel in overview.

Also fixes two pre-existing test issues:
- overview.spec.ts: active gateway radio is intentionally enabled
  (users need to re-trigger switch for the already-selected gateway)
- drain.spec.ts: stubs drain status via page.route() since Docker
  has no real conntrack entries; fixed Playwright glob pattern bug
  where **/ubus* fails to match /ubus/?timestamp (regex needed)

root/etc/config/zt-gateway

@@ -4,6 +4,14 @@ config global 'global'
 	option drain_timeout '600'
 	option health_interval '60'
 	option auto_failback '0'
+	option wiblan_subnet '10.11.13.0/24'
+	option bridge_device 'br-zt'
+	option bridge_ports 'ztabc0'
+	option wiblan_gw '10.11.13.1'
+	option table_main '100'
+	option table_drain '101'
+	option dhcp_lease_first '10.11.13.100'
+	option dhcp_lease_last '10.11.13.200'
 
 config gateway
 	option region 'amsterdam'