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/appointments/page.tsx

@@ -7,23 +7,11 @@ import { api } from "../../api";
 import { Header } from "../../components/Header";
 import { useRequireAuth } from "../../hooks/useRequireAuth";
 import { components } from "../../generated/api";
+import { formatDateTime } from "../../utils/date";
 
 type AppointmentResponse = components["schemas"]["AppointmentResponse"];
 type PaginatedAppointments = components["schemas"]["PaginatedResponse_AppointmentResponse_"];
 
-// Helper to format datetime
-function formatDateTime(isoString: string): string {
-  const d = new Date(isoString);
-  return d.toLocaleString("en-US", {
-    weekday: "short",
-    month: "short",
-    day: "numeric",
-    hour: "2-digit",
-    minute: "2-digit",
-    hour12: false,
-  });
-}
-
 // Helper to get status display
 function getStatusDisplay(status: string): { text: string; bgColor: string; textColor: string } {
   switch (status) {