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/test-results/.last-run.json

@@ -1,7 +1,4 @@
 {
-  "status": "failed",
-  "failedTests": [
-    "647d672ac99574a52088-7123e0baf27b194c0b82",
-    "647d672ac99574a52088-a2e9f69e9c9ef92dc2bb"
-  ]
+  "status": "passed",
+  "failedTests": []
 }