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/store/store_test.go

@@ -35,7 +35,7 @@ func TestStateHistoryAndLeases(t *testing.T) {
 	sn := &model.Subnet{ID: ids[0], Name: "office", Interface: "eth0", Prefix: netip.MustParsePrefix("10.0.10.0/24"),
 		Pools:   []model.Pool{{Start: netip.MustParseAddr("10.0.10.100"), End: netip.MustParseAddr("10.0.10.199")}},
 		Options: []model.OptionValue{{Code: 3, Value: "10.0.10.1"}}}
-	r := &model.Reservation{ID: ids[1], MAC: "aa:bb:cc:dd:ee:01", IP: netip.MustParseAddr("10.0.10.20"), Hostname: "nas"}
+	r := &model.Reservation{ID: ids[1], MAC: "aa:bb:cc:dd:ee:01", IP: netip.MustParseAddr("10.0.10.20"), Hostname: "nas", Disabled: true}
 	set := st.Settings
 	set.LeaseTime = 3600
 	e := &model.HistoryEntry{Time: time.Now(), Username: "admin", Via: "cli", Description: "add office",
@@ -51,7 +51,7 @@ func TestStateHistoryAndLeases(t *testing.T) {
 	if len(st.Subnets) != 1 || st.Subnets[0].Pools[0].Size() != 100 || st.Subnets[0].Options[0].Value != "10.0.10.1" {
 		t.Errorf("subnets %+v", st.Subnets)
 	}
-	if len(st.Reservations) != 1 || st.Reservations[0].Hostname != "nas" || st.Settings.LeaseTime != 3600 {
+	if len(st.Reservations) != 1 || st.Reservations[0].Hostname != "nas" || !st.Reservations[0].Disabled || st.Settings.LeaseTime != 3600 {
 		t.Errorf("reservations %+v settings %+v", st.Reservations, st.Settings)
 	}
 	// Undo: the inverse changes, marked as a revert.