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)
