fix: resolve ESLint errors blocking Docker build

- Remove unused formatDate() from park page (replaced by formatShortDate)
- Remove unused date prop from DayCell component
- Change let to const for cellBorderRadius in ParkCard
- Change let to const for bg in ParkMonthCalendar

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

app/park/[id]/page.tsx

@@ -121,12 +121,6 @@ export default async function ParkPage({ params, searchParams }: PageProps) {
 
 // ── Helpers ────────────────────────────────────────────────────────────────
 
-function formatDate(iso: string): string {
-  return new Date(iso + "T00:00:00").toLocaleDateString("en-US", {
-    weekday: "long", month: "long", day: "numeric",
-  });
-}
-
 function formatShortDate(iso: string): string {
   return new Date(iso + "T00:00:00").toLocaleDateString("en-US", {
     weekday: "short", month: "short", day: "numeric",