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/ParkMonthCalendar.tsx

@@ -139,7 +139,7 @@ export function ParkMonthCalendar({ parkId, year, month, monthData, today }: Par
               const isOpen = dayData?.isOpen && dayData?.hoursLabel;
               const isPH = dayData?.specialType === "passholder_preview";
 
-              let bg = isToday
+              const bg = isToday
                 ? "var(--color-today-bg)"
                 : isWeekend
                 ? "var(--color-weekend-header)"