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/WeekCalendar.tsx

@@ -28,12 +28,10 @@ function parseDate(iso: string) {
 }
 
 function DayCell({
-  date,
   dayData,
   isToday,
   isWeekend,
 }: {
-  date: string;
   dayData: DayData | undefined;
   isToday: boolean;
   isWeekend: boolean;
@@ -214,7 +212,6 @@ function ParkRow({
       {weekDates.map((date, i) => (
         <DayCell
           key={date}
-          date={date}
           dayData={parkData[date]}
           isToday={date === today}
           isWeekend={parsedDates[i].isWeekend}