Month grid · 7 columns

Build a clean month view as an `App` React component.

- render a `.month` container holding 35 `.cell` divs (5 weeks × 7 days)
- each cell shows the day number in a `.d` span
- arrange cells in a 7-column grid, ~56px tall, 1px gap
- cells render the day number top-left

Use the day-number formula in the starter so the dates roll correctly.