Add heatmap js and css

.gitignore

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+test.html
+activity.json

assets/css/heatmap.css

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+:root{
+    --color-empty: #F5F5F5;
+    --color-low: #BDD7EE;
+    --color-mid: #2E86C1;
+    --color-high: #1A5276
+}

assets/js/build_heatmap.js

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+export function flattenData(raw) {
+  const result = {};
+  for (const [date, repos] of Object.entries(raw)) {
+    result[date] = Object.values(repos).reduce((sum, n) => sum + n, 0);
+  }
+  return result;
+}
+
+export function constructWeeks() {
+  const today = new Date();
+  today.setHours(0, 0, 0, 0);
+
+  const start = new Date(today);
+  start.setDate(today.getDate() - 52 * 7);
+  start.setDate(start.getDate() - start.getDay());
+
+  const weeks = [];
+  const cur = new Date(start);
+  while (cur <= today) {
+    const week = [];
+    for (let d = 0; d < 7; d++) {
+      const day = new Date(cur);
+      week.push(day <= today ? day : null);
+      cur.setDate(cur.getDate() + 1);
+    }
+    weeks.push(week);
+  }
+  return weeks;
+}
+
+function getColor(count) {
+  if (count === 0) return "var(--color-empty)";
+  if (count <= 3) return "var(--color-low)";
+  if (count <= 10) return "var(--color-mid)";
+  return "var(--color-high)";
+}
+
+export function render(weeks, counts) {
+  const CELL = 13;
+  const GAP = 2;
+  const SHIFT = CELL + GAP;
+
+  const svgNS = "http://www.w3.org/2000/svg";
+  const svg = document.createElementNS(svgNS, "svg");
+
+  svg.setAttribute("width", weeks.length * SHIFT + 30);
+  svg.setAttribute("height", SHIFT * 7 + 20);
+
+  weeks.forEach((week, col) => {
+    week.forEach((day, row) => {
+      if (day === null) {
+        return;
+      }
+      const key = day.toISOString().slice(0, 10); // Get the key in yyyy-mm-dd format
+
+      const count = counts[key] || 0;
+
+      const rect = document.createElementNS(svgNS, "rect");
+      rect.setAttribute("x", col * SHIFT + 30);
+      rect.setAttribute("y", row * SHIFT);
+      rect.setAttribute("height", CELL);
+      rect.setAttribute("width", CELL);
+      rect.setAttribute("fill", getColor(count));
+      rect.setAttribute("rx", 2);
+      rect.setAttribute("data-date", key);
+      rect.setAttribute("data-count", count);
+
+      svg.appendChild(rect);
+    });
+
+    document.getElementById("heatmap").appendChild(svg);
+  });
+}