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_config.go

@@ -132,7 +132,7 @@ func (s *Server) presentSubnet(sn *model.Subnet, st *model.State, leases []*mode
 	}
 	reserved := map[netip.Addr]bool{}
 	for _, res := range st.Reservations {
-		if sn.Prefix.Contains(res.IP) {
+		if sn.Prefix.Contains(res.IP) && !res.Disabled {
 			out.Usage.Reservations++
 			reserved[res.IP] = true
 			for _, p := range sn.Pools {
@@ -421,6 +421,9 @@ func applyReservationInput(res *model.Reservation, in *api.ReservationInput) err
 		}
 		res.Options = opts
 	}
+	if in.Disabled != nil {
+		res.Disabled = *in.Disabled
+	}
 	if in.Comment != nil {
 		res.Comment = *in.Comment
 	}