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/web/static/assets/style.css

@@ -110,11 +110,9 @@ header.top .spacer { flex: 1; }
 header.top form.search input { width: 220px; }
 header.top .user { color: var(--muted); }
 main { padding: 16px; max-width: 1400px; margin: 0 auto; }
-  display: flex; align-items: center; gap: 12px; flex-wrap: wrap;
-  background: var(--warn-soft); color: var(--text); border-bottom: 1px solid var(--border);
-  padding: 8px 16px;
-}
-.page-head { display: flex; align-items: flex-start; gap: 12px; flex-wrap: wrap; margin-bottom: 12px; }
+.page-head { display: flex; align-items: flex-start; gap: 12px; flex-wrap: wrap; margin-bottom: 16px; }
+.page-head > .actions { padding: 4px 0; }
+.filter-row + .panel, .actions + .panel { margin-top: 4px; }
 .page-head .grow { flex: 1; min-width: 200px; }
 .actions { display: flex; gap: 8px; flex-wrap: wrap; align-items: center; }