Fix: Update permissions and add missing /api/exchange/slots endpoint

- Updated auth-context.tsx to use new exchange permissions
  (CREATE_EXCHANGE, VIEW_OWN_EXCHANGES, etc.) instead of old
  appointment permissions (BOOK_APPOINTMENT, etc.)

- Updated exchange/page.tsx, trades/page.tsx, admin/trades/page.tsx
  to use correct permission constants

- Updated profile/page.test.tsx mock permissions

- Updated admin/availability/page.tsx to use constants.exchange
  instead of constants.booking

- Added /api/exchange/slots endpoint to return available slots
  for a date, filtering out already booked slots

- Fixed E2E tests:
  - exchange.spec.ts: Wait for button to be enabled before clicking
  - permissions.spec.ts: Use more specific heading selector
  - price-history.spec.ts: Expect /exchange redirect for regular users

frontend/app/admin/availability/page.tsx

@@ -22,7 +22,7 @@ import {
   modalStyles,
 } from "../../styles/shared";
 
-const { slotDurationMinutes, maxAdvanceDays, minAdvanceDays } = constants.booking;
+const { slotDurationMinutes, maxAdvanceDays, minAdvanceDays } = constants.exchange;
 
 type _AvailabilityDay = components["schemas"]["AvailabilityDay"];
 type AvailabilityResponse = components["schemas"]["AvailabilityResponse"];