docs: document v6 editor-chrome regressions and update polish plan priorities

- Add 5 new critical regressions found during thorough editor-chrome evaluation:
  - Notification button navigates to `/notifications` page instead of opening inline popup (no NotificationPopup component exists)
  - Right sidebar "Page/Note/Arrow Properties" header scrolls away with content instead of staying fixed
  - "N items selected" indicator in normal document flow shrinks canvas (should be absolute positioned)
  - Canvas

Author: gustavotoyota