Onboarding overlay subsumes the boot card now that it mounts from frame 1 and renders boot progress inline. The standalone DesktopBootOverlay is unreachable in every flow (yields whenever onboarding has not confirmed configured, dismisses once it has).
Show the desktop provider onboarding flow before prompt submission when no inference provider is configured, preventing fresh installs from falling through to backend credential errors.
Promote closeRightRailTab/closeActiveRightRailTab as the single
public entry point. Drops the activeTabRef + handleCloseDocument
indirection in ChatPreviewRail, the unused $rightRailHasContent
atom, and the legacy dismissFilePreviewTarget alias. -70 LOC.