The storage classes sidebar entry is a button, not a link. Using getByRole('link')
caused the test to fail with 'element not found'. Switch to getByRole('button')
which matches the actual Headlamp sidebar structure.
Also increased timeout from 10s to 15s for consistency with other waits.
Co-Authored-By: Paperclip <noreply@paperclip.ing>
Add waitForSidebar helper function with explicit sidebar visibility wait
and networkidle state to ensure page is fully loaded before assertions.
This addresses flaky E2E tests where elements were not consistently
found due to timing issues during page transitions.