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/e2e/appointments.spec.ts

@@ -178,8 +178,8 @@ test.describe("Appointments Page - With Bookings", () => {
     // Click No to abort
     await page.getByRole("button", { name: "No" }).click();
     
-    // Should still show as booked
-    await expect(page.getByText("Booked", { exact: true })).toBeVisible();
+    // Should still show as booked (use first() since there may be multiple bookings)
+    await expect(page.getByText("Booked", { exact: true }).first()).toBeVisible();
   });
 });