IAV-1153: fix z-index stacking, snackbar position, account persistence and wizard init

Summary

• CustomSelect portal: The dropdown is now rendered via createPortal at document.body level with position: fixed, completely escaping any stacking context hierarchy. This fixes the dropdown being hidden behind wizard navigation buttons. Click-outside detection was updated to exclude the portal element, fixing option selection being swallowed before onClick could fire.

• Aqal wizard – integrations config:

  • Snackbar (success & error) moved from bottom-right to top-right
  • Channel select dropdowns (openUpward removed) now open downward
  • "Modifier" button no longer clears the selected account — the account stays pre-selected so the user doesn't have to re-select it