Dashboard Cells

Server-rendered cell HTML for specific blocks — the live-add twin of the page render. The dashboard page calls this when a block is added (by the agent or a collaborator), inserts the returned cell, and applies grid_template_rows to reflow the grid. One mold: it renders the same _dash_block_cell.html partial the full page uses, so an added cell is identical to existing ones. Auth mirrors the page route — 404 on no access, never leak existence.