Stats: match GitHub's contribution-graph look

- GitHub's green scale (#0e4429 -> #39d353) instead of the orange
  accent-mix
- Mon/Wed/Fri row labels down the left (right-aligned, muted),
  month-label row offset to line up with the grid
- legend right-aligned under the grid

static/app.js

@@ -487,6 +487,10 @@ function heatMap(byDay, days) {
               "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"];
   const grid = el("div", "heat");
   const lbl = el("div", "heat-lbl");
+  const dcol = el("div", "heat-days");
+  for (const t of ["Mon", "", "Wed", "", "Fri", "", ""]) dcol.append(el("span", null, t));
+  const row = el("div", "heat-row");
+  row.append(dcol, grid);
   let lastM = -1;
   for (let d = new Date(start); d <= today; ) {
     const col = el("div", "heat-col");
@@ -523,7 +527,7 @@ function heatMap(byDay, days) {
     lg.append(s);
   }
   lg.append(el("span", "muted", "more"));
-  return [lbl, grid, lg];
+  return [lbl, row, lg];
 }
 
 function heatSection() {