Disable reservations; subnet enable/disable buttons; page-head CSS fix

- A disabled reservation is kept but ignored: the client gets a pool
  address like any other (steward reservation disable|enable, buttons and
  a checkbox in the web UI). Schema migration 3.
- Subnets get Disable/Enable buttons on their page and in the list.
- A rule left without its selector in style.css broke the page heading's
  layout, so the New ... buttons sat cramped under the description.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/server/h_leases.go

@@ -29,7 +29,7 @@ func (s *Server) presentLease(l *model.Lease, st *model.State) *api.Lease {
 		}
 	}
 	for _, r := range st.Reservations {
-		if r.IP == l.IP {
+		if r.IP == l.IP && !r.Disabled {
 			out.Reservation, out.ReservedHostname = r.ID, r.Hostname
 		}
 	}