fix(hosts): the Add host dialog sits centred, not in the top-left corner (#5291)

The stylesheet's reset (* { margin: 0 }) removed the browser's margin: auto
that centres a modal <dialog>; .modal puts it back.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

steward/static/steward.css

@@ -311,8 +311,9 @@ a.stat-tile:hover { border-color: var(--border-mid); background: var(--bg-elevat
 .host-pair { display: grid; grid-template-columns: repeat(auto-fit, minmax(min(420px, 100%), 1fr)); gap: 1rem; margin-bottom: 1rem; }
 .host-pair > * { min-width: 0; }
 .host-pair .card { margin-bottom: 0; height: 100%; }
-/* A small form over the page (native <dialog>; Esc closes it). */
-.modal { width: min(560px, calc(100vw - 2rem)); padding: 1.25rem; color: var(--text); background: var(--bg-card);
+/* A small form over the page (native <dialog>; Esc closes it). Centred: the
+   reset at the top (* { margin: 0 }) takes away the browser's margin: auto. */
+.modal { margin: auto; width: min(560px, calc(100vw - 2rem)); padding: 1.25rem; color: var(--text); background: var(--bg-card);
   border: 1px solid var(--border); border-radius: var(--fs-radius-md); box-shadow: var(--fs-shadow-3); }
 .modal::backdrop { background: var(--fs-overlay); }
 .modal-title { font-family: var(--font-serif); font-size: 1.25rem; font-weight: 500; margin: 0 0 1rem; }