feat: restructure client-side code

public/utils/colors.js

@@ -0,0 +1,11 @@
+// Stable distinct color per year for the year badge
+export function badgeColorForYear(y) {
+  const val = (y === undefined || y === null) ? '?' : y;
+  if (val === '?' || Number.isNaN(Number(val))) {
+    // Neutral slate for unknown years
+    return 'background-color: hsl(215 16% 34%);';
+  }
+  const n = Number(val);
+  const hue = ((n * 23) % 360 + 360) % 360; // spread hues deterministically
+  return `background-color: hsl(${hue} 70% 42%);`;
+}