Add DHCP reservations to the web UI, split out a fast webui test

The DHCP page edits the reservations per LAN (checked like the module's
assertions) and offers active clients for reservation. The web UI checks
that need no installed system moved from apply.nix into webui.nix (one
node, under a minute); apply.nix shares one apply for the pool change and
a reservation.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AeFqV9dZrK6nipHkPfyM4F

scripts/test.sh

@@ -6,7 +6,7 @@
 #                              and test_script() work; boot once, retry steps
 #   router-test --list
 # Logs: ${TMPDIR:-/tmp}/router-test-logs/NAME.log
-vm_tests="basic pppoe vlans onestick rollback dns installer apply setup update image wireguard"
+vm_tests="webui basic pppoe vlans onestick rollback dns installer apply setup update image wireguard"
 flake=${FLAKE:-.}
 logs=${TMPDIR:-/tmp}/router-test-logs
 mkdir -p "$logs"