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/api/types.go

@@ -90,6 +90,7 @@ type ReservationInput struct {
 	IP       *string        `json:"ip,omitempty"`
 	Hostname *string        `json:"hostname,omitempty"`
 	Options  *[]OptionInput `json:"options,omitempty"`
+	Disabled *bool          `json:"disabled,omitempty"`
 	Comment  *string        `json:"comment,omitempty"`
 }