refactor: store selected week in a cookie, not the URL

The home page no longer reads ?week=YYYY-MM-DD from the URL. Selected week
lives in the tcWeek cookie, set via a server action that revalidates the
home page so the next render reflects it. The URL stays at "/" regardless
of which week the user is viewing.

WeekNav prev/next/today buttons (and the arrow-key bindings) call the
server action directly — no router.refresh dance, no client-side cookie
write. BackToCalendarLink drops its localStorage-based href reconstruction
and just links to "/" since the cookie already remembers the right week
across navigations.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

components/HomePageClient.tsx

@@ -109,11 +109,6 @@ export function HomePageClient({
     return () => timeouts.forEach(clearTimeout);
   }, [isCurrentWeek, today, data, router]);
 
-  // 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);