test: improve e2e tests and add COMPLETE_EXCHANGE permission tests

- Fix E2E test assertion for buy/sell direction change
- Add data-testid to date buttons for reliable e2e selection
- Update e2e tests to use data-testid instead of fragile weekday matching
- Add tests for regular user cannot complete/no-show trades (COMPLETE_EXCHANGE permission)

frontend/app/exchange/page.tsx

@@ -462,6 +462,7 @@ export default function ExchangePage() {
                   return (
                     <button
                       key={dateStr}
+                      data-testid={`date-${dateStr}`}
                       onClick={() => handleDateSelect(date)}
                       disabled={isDisabled}
                       style={{