Year heatmap

Build a GitHub-style contribution heatmap.

- 53 columns × 7 rows (one cell per day of the year)
- cells are square, ~11px, 2px gap
- column-major layout — top-to-bottom first, then next column
- tint by `data-level` (0–4); 0 = empty, 4 = brightest

Palette: 0 #1a1d22 · 1 #0e3a1f · 2 #156d2e · 3 #2ea043 · 4 #56d364