feat: building purchase particle burst effects (#57)

Adds DOM-based particle burst animations when buying buildings and
completing research projects. Blue particles for buildings, gold for
projects. Lightweight CSS animation with no external dependencies.

Refs #57 — Night of Polish, Task 1 (Visual Identity)

js/utils.js

@@ -285,6 +285,31 @@ function getClickPower() {
     return (1 + Math.floor(G.buildings.autocoder * 0.5) + Math.max(0, (G.phase - 1)) * 2) * G.codeBoost;
 }
 
+/**
+ * Spawns a burst of particles at (x, y) for visual feedback.
+ * @param {number} x - Center X in viewport pixels.
+ * @param {number} y - Center Y in viewport pixels.
+ * @param {string} color - Particle color (CSS value).
+ * @param {number} [count=12] - Number of particles.
+ */
+function spawnParticles(x, y, color, count) {
+    count = count || 12;
+    for (let i = 0; i < count; i++) {
+        const el = document.createElement('div');
+        el.className = 'particle';
+        const size = 3 + Math.random() * 4;
+        const angle = (Math.PI * 2 * i / count) + (Math.random() - 0.5) * 0.5;
+        const dist = 30 + Math.random() * 40;
+        const px = Math.cos(angle) * dist;
+        const py = Math.sin(angle) * dist;
+        el.style.cssText =
+            'left:' + x + 'px;top:' + y + 'px;width:' + size + 'px;height:' + size +
+            'px;background:' + color + ';--px:' + px + 'px;--py:' + py + 'px';
+        document.body.appendChild(el);
+        setTimeout(function() { el.remove(); }, 650);
+    }
+}
+
 /**
  * Calculates production rates for all resources based on buildings and boosts.
  */