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)

docker-compose.yml

@@ -26,6 +26,7 @@ services:
       - ./root/usr/share/ucode/luci/runtime.uc:/usr/share/ucode/luci/runtime.uc:ro
       - ./root/usr/share/luci/menu.d/luci-app-zt-gateway.json:/usr/share/luci/menu.d/luci-app-zt-gateway.json:ro
       - ./root/usr/share/rpcd/acl.d/luci-app-zt-gateway.json:/usr/share/rpcd/acl.d/luci-app-zt-gateway.json:ro
+      - ./root/usr/sbin/zt-gateway-setup:/usr/sbin/zt-gateway-setup:ro
       # Stage host config so the entrypoint can copy it into the writable overlay.
       - ./root/etc/config:/host-config:ro
       - ./htdocs/luci-static/resources/view/zt-gateway/overview.js:/www/luci-static/resources/view/zt-gateway/overview.js:ro