Onboarding structure preview now shows Spaces as a column grid

During onboarding, the agent now renders its proposed workspace as a side-by-side column grid — one column per Space — each showing the Space description and a flat topic list. The segmentation question ("does this split make sense?") is front and center, so it's easy to see at a glance how your knowledge would be divided before anything is written. Edits are conversational: just say "rename X", "merge these two Spaces", or "add a topic" and the agent adjusts the preview before building anything.