The X button in the footer's Background Tasks popover silently failed when
ai_sessions.lockedByTab pointed to a stale/closed tab: cancelPlanning et al.
returned 409, dismissSession set lockConflict and bailed before calling
deleteAiSession. Always proceed to delete; the dismissal tombstone already
prevents stale SSE/sync from resurrecting the session.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>