Fix the order:hot default: seed the toggle and clear stale defaults

Two bugs made the toggle look broken even though the preference was
stored correctly (JakeBreathild had online_hot_default false):

- The Account card never seeded online_hot_default into its form state,
  so the checkbox always rendered checked via the '?? true' fallback.
  It now starts from the saved value.
- Turning the toggle off did not change Online when the URL still
  carried tags=order:hot (e.g. Ctrl+Shift+R reloading the old URL).
  order:hot on its own is the default, not a deliberate search, so it is
  now removed when the account has the toggle off; 'order:hot canine' or
  any other search is still left untouched. The decision moved into
  hotDefault.ts (set-hot / clear-hot / keep) with the matrix verified in
  Node.

frontend/src/features/account/PreferencesCard.tsx

@@ -25,6 +25,7 @@ export function PreferencesCard() {
     ordering: user?.preferences?.ordering ?? "-created_at",
     per_page: user?.preferences?.per_page ?? 48,
     zoom: user?.preferences?.zoom ?? 190,
+    online_hot_default: user?.preferences?.online_hot_default ?? true,
   });
   const [saving, setSaving] = useState(false);