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

pkgs/router-webui/system.go

@@ -20,6 +20,15 @@ type System struct {
 	StateDir  string // /var/lib/router: secrets, UI state
 	// ReleaseFile describes the installed nixos-router; /etc/router-release if empty.
 	ReleaseFile string
+	// LeasesDNSDir is where leases-dns leaves its status; /run/router-leases-dns if empty.
+	LeasesDNSDir string
+}
+
+func (s System) leasesDNSDir() string {
+	if s.LeasesDNSDir != "" {
+		return s.LeasesDNSDir
+	}
+	return "/run/router-leases-dns"
 }
 
 func (s System) settingsPath() string  { return filepath.Join(s.ConfigDir, "settings.json") }