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.
The reusable plugin-e2e.yaml@main lacks pnpm support. Switching to
the PR branch that has pnpm detector, Corepack setup, and pnpm commands.
Will revert to @main once PR #141 merges.
- PRI-619 E2E fix
Co-Authored-By: Paperclip <noreply@paperclip.ing>