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>

scripts/e2e.sh

@@ -106,6 +106,15 @@ run "client on eth1 gets the other subnet" 0 udhcpc b 02:00:00:00:00:02 -x hostn
 contains "BOUND ip=$NB.100 mask=255.255.255.0 router=$NB.1 dns= domain=lan lease=3600 serverid=$NB.2"
 run "reserved client gets its address, host name and option" 0 udhcpc a 02:00:00:00:00:20
 contains "BOUND ip=$NA.20 .*hostname=printer ntp=$NA.2"
+run "disable the reservation" 0 S reservation disable $NA.20
+run "a disabled reservation's client gets a pool address" 0 udhcpc a 02:00:00:00:00:20
+contains "BOUND ip=$NA.10"
+not_contains "hostname=printer"
+POOLIP=$(echo "$OUT" | sed -n 's/^BOUND ip=\([0-9.]*\) .*/\1/p')
+run "enable it again" 0 S reservation enable $NA.20
+run "the reserved address again" 0 udhcpc a 02:00:00:00:00:20
+contains "BOUND ip=$NA.20 .*hostname=printer"
+run "free the pool address it had meanwhile" 0 S lease delete "$POOLIP" --yes
 run "the same device on the other VLAN gets that VLAN's address" 0 udhcpc b 02:00:00:00:00:20
 contains "BOUND ip=$NB.101 "
 run "leases" 0 S lease list