refactor: separate js code

js/particle.js

@@ -0,0 +1,28 @@
+class Particle {
+  constructor(x, y, color) {
+    this.x = x;
+    this.y = y;
+    this.vx = (Math.random() - 0.5) * S.particleVel;
+    this.vy = (Math.random() - 0.5) * S.particleVel;
+    this.life = 1;
+    this.decay = S.particleDecayBase + Math.random() * S.particleDecayRange;
+    this.color = color;
+    this.size = S.particleSizeMin + Math.random() * (S.particleSizeMax - S.particleSizeMin);
+  }
+  update() {
+    this.x += this.vx;
+    this.y += this.vy;
+    this.life -= this.decay;
+    this.vx *= S.particleDamping;
+    this.vy *= S.particleDamping;
+  }
+  draw() {
+    ctx.save();
+    ctx.globalAlpha = this.life;
+    ctx.fillStyle = this.color;
+    ctx.shadowColor = this.color;
+    ctx.shadowBlur = S.particleGlow;
+    ctx.fillRect(this.x - this.size / 2, this.y - this.size / 2, this.size, this.size);
+    ctx.restore();
+  }
+}