fix: "← Calendar" returns to the previously viewed week

HomePageClient writes the current weekStart to localStorage("lastWeek")
whenever it changes. BackToCalendarLink (new client component) reads
that value on mount and builds the href — falling back to "/" if nothing
is stored (e.g. direct link to a park page).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

components/HomePageClient.tsx

@@ -41,6 +41,11 @@ export function HomePageClient({
     setCoastersOnly(localStorage.getItem(COASTER_MODE_KEY) === "true");
   }, []);
 
+  // Remember the current week so the park page back button returns here.
+  useEffect(() => {
+    localStorage.setItem("lastWeek", weekStart);
+  }, [weekStart]);
+
   const toggle = () => {
     const next = !coastersOnly;
     setCoastersOnly(next);