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/dhcpd/config.go

@@ -52,6 +52,9 @@ func compile(st *model.State) *config {
 		c.byIface[s.Interface] = append(c.byIface[s.Interface], sc)
 	}
 	for _, r := range st.Reservations {
+		if r.Disabled {
+			continue
+		}
 		c.resByIP[r.IP] = r
 		s := st.SubnetFor(r.IP)
 		if s == nil {