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/online/hotDefault.ts

@@ -0,0 +1,22 @@
+/** The metatag the Online page opens with when the account toggle is on. */
+export const HOT_ORDER = "order:hot";
+
+export type HotDefaultAction = "set-hot" | "clear-hot" | "keep";
+
+/**
+ * Decide what the Online default should do for a given search.
+ *
+ * `order:hot` on its own is the default, not a deliberate search: it is
+ * applied when the toggle is on and removed when the account turns it off
+ * (so a reload of an old URL follows the new setting). Any other search is
+ * left untouched.
+ */
+export function hotDefaultAction(
+  tags: string,
+  hotDefaultOn: boolean,
+): HotDefaultAction {
+  const trimmed = tags.trim();
+  if (trimmed === HOT_ORDER) return hotDefaultOn ? "keep" : "clear-hot";
+  if (!trimmed) return hotDefaultOn ? "set-hot" : "keep";
+  return "keep";
+}