Extract duplicate date formatting functions to shared utilities

- Created frontend/app/utils/date.ts with formatDate, formatTime, formatDateTime, formatDisplayDate
- Created frontend/e2e/helpers/date.ts with formatDateLocal, getTomorrowDateStr
- Updated all frontend pages and e2e tests to use shared utilities
- Removed duplicate date formatting code from 6 files

frontend/app/admin/availability/page.tsx

@@ -8,6 +8,7 @@ import { Header } from "../../components/Header";
 import { useRequireAuth } from "../../hooks/useRequireAuth";
 import { components } from "../../generated/api";
 import constants from "../../../../shared/constants.json";
+import { formatDate, formatDisplayDate } from "../../utils/date";
 
 const { slotDurationMinutes, maxAdvanceDays } = constants.booking;
 
@@ -15,14 +16,6 @@ type AvailabilityDay = components["schemas"]["AvailabilityDay"];
 type AvailabilityResponse = components["schemas"]["AvailabilityResponse"];
 type TimeSlot = components["schemas"]["TimeSlot"];
 
-// Helper to format date as YYYY-MM-DD in local timezone
-function formatDate(d: Date): string {
-  const year = d.getFullYear();
-  const month = String(d.getMonth() + 1).padStart(2, "0");
-  const day = String(d.getDate()).padStart(2, "0");
-  return `${year}-${month}-${day}`;
-}
-
 // Helper to get next N days starting from tomorrow
 function getDateRange(): Date[] {
   const dates: Date[] = [];
@@ -223,10 +216,6 @@ export default function AdminAvailabilityPage() {
     }
   };
 
-  const formatDisplayDate = (d: Date): string => {
-    return d.toLocaleDateString("en-US", { weekday: "short", month: "short", day: "numeric" });
-  };
-
   const formatSlotTime = (slot: TimeSlot): string => {
     return `${slot.start_time.slice(0, 5)} - ${slot.end_time.slice(0, 5)}`;
   };