feat: restructure client-side code

public/client.js

@@ -1,3 +1,5 @@
+import { badgeColorForYear } from './utils/colors.js';
+
 let ws;
 let reconnectAttempts = 0;
 let reconnectTimer = null;
@@ -233,17 +235,7 @@ function shortName(id) {
 
 function escapeHtml(s) { return String(s).replace(/[&<>"']/g, c => ({'&':'&amp;','<':'&lt;','>':'&gt;','"':'&quot;',"'":'&#39;'}[c])); }
 
-// Stable distinct color per year for the year badge
-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%);`;
-}
+// color helper imported from utils/colors.js
 
 function handleMessage(ev) {
   const msg = JSON.parse(ev.data);