Publish DHCP clients' names in DNS

router-webui leases-dns reads Kea's leases and publishes <name>.<domain>
with reverse records in the running Unbound (router.dns.dynamicLeases).
Names the configuration publishes are never taken over; a client's name
is cut to its first label and kept inside the local domain; of two
clients the newer lease wins. The DNS page switches it on and shows what
is published and what was skipped; router-setup enables it with a domain.

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

tests/setup.nix

@@ -145,7 +145,7 @@ pkgs.testers.runNixOSTest {
         assert r["wan"]["ipv6"] == {"enable": True, "transport": "ppp"}, s
         assert r["lans"]["lan"]["interface"] == "eth1" and r["lans"]["lan"]["ipv6"] == {"pd": True}, s
         assert "vlan" not in r["lans"]["lan"], s
-        assert r["dns"]["domain"] == "home.test", s
+        assert r["dns"] == {"domain": "home.test", "dynamicLeases": True}, s
         assert "pppoe-secret" not in s
         installer.succeed("grep -qx pppoe-secret /tmp/vlan-config/pppoe-password")
         installer.fail("blkid /dev/vda1")