Sentry dashboard 6-column grid

Use Sentry’s 6-column dashboard grid: big_number is 2×1; line, area, and bar are 3×2; table is 6×2 and always occupies its own row. stacked_area, top_n, categorical_bar, text, details, wheel, rage_and_dead_clicks, server_tree, and agents_traces_table are 3×2. Chose these fixed dimensions over ad hoc sizing because they preserve predictable row packing.