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);
 

frontend/src/features/online/OnlinePage.tsx

@@ -17,6 +17,7 @@ import {
   postIsBlacklisted,
   type E621Post,
 } from "@/lib/e621";
+import { hotDefaultAction, HOT_ORDER } from "@/features/online/hotDefault";
 import { useE621 } from "@/store/e621";
 import { useAuth } from "@/store/auth";
 import { useUi } from "@/store/ui";
@@ -44,14 +45,21 @@ export default function OnlinePage() {
   const zoom = useUi((state) => state.zoom);
   const appliedHotDefault = useRef(false);
 
-  // Landing on Online without a search uses the order:hot default (on unless
-  // the account turns it off). Existing searches are left alone.
+  // Landing on Online follows the account's order:hot default: applied when
+  // there is no search, removed when the saved URL only carries the default
+  // and the toggle was turned off. Deliberate searches are left alone.
   useEffect(() => {
     if (!authReady || appliedHotDefault.current) return;
     appliedHotDefault.current = true;
-    if (tags) return;
-    if (user?.preferences?.online_hot_default === false) return;
-    setSearchParams(buildParams("order:hot", 1), { replace: true });
+    const action = hotDefaultAction(
+      tags,
+      user?.preferences?.online_hot_default !== false,
+    );
+    if (action === "set-hot") {
+      setSearchParams(buildParams(HOT_ORDER, 1), { replace: true });
+    } else if (action === "clear-hot") {
+      setSearchParams(buildParams("", 1), { replace: true });
+    }
   }, [authReady, tags, user, setSearchParams]);
 
   const blacklistMutation = useMutation({

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";
+}