Fix flaky E2E tests with proper selectors and response waits

- Use data-testid attributes to target specific day cards in availability tests
- Wait for networkidle before interacting with page elements
- Set up PUT and GET response listeners before triggering actions
- Add retry logic for availability API in booking tests
- Fix appointments test to handle multiple 'Booked' elements with .first()
- Increase parallel workers to 12 for faster test execution

frontend/app/admin/availability/page.tsx

@@ -286,6 +286,7 @@ export default function AdminAvailabilityPage() {
               return (
                 <div
                   key={dateStr}
+                  data-testid={`day-card-${dateStr}`}
                   style={{
                     ...styles.dayCard,
                     ...(hasSlots ? styles.dayCardActive : {}),