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/model/config.go

@@ -195,7 +195,10 @@ type Reservation struct {
 	// Hostname is sent to the client (option 12).
 	Hostname string        `json:"hostname"`
 	Options  []OptionValue `json:"options"`
-	Comment  string        `json:"comment"`
+	// Disabled reservations are kept but ignored: the client is treated
+	// like any other, and an address inside a pool may go to others.
+	Disabled bool   `json:"disabled"`
+	Comment  string `json:"comment"`
 }
 
 // Label names the reservation in messages.