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.go

@@ -193,6 +193,10 @@ CREATE INDEX audit_time ON audit (time);
 ALTER TABLE tokens ADD COLUMN rotated_at TEXT;
 ALTER TABLE tokens ADD COLUMN prev_secret_hash TEXT NOT NULL DEFAULT '';
 ALTER TABLE tokens ADD COLUMN prev_expires_at TEXT;
+`,
+	// 3: disabled reservations.
+	`
+ALTER TABLE reservations ADD COLUMN disabled INTEGER NOT NULL DEFAULT 0;
 `,
 }