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>

components/ParkCard.tsx

@@ -57,7 +57,7 @@ export function ParkCard({ park, weekDates, parkData, today }: ParkCardProps) {
 
           let cellBg = "transparent";
           let cellBorder = "1px solid var(--color-border-subtle)";
-          let cellBorderRadius = "6px";
+          const cellBorderRadius = "6px";
 
           if (isToday) {
             cellBg = "var(--color-today-bg)";