Particules interactives

// Particules interactives — gravité N-body simplifiée
particles.forEach(p => {
  const dx = mx - p.x, dy = my - p.y;
  const dist = Math.sqrt(dx * dx + dy * dy) + 1;
  const f = force / (dist * 0.5);

  // Attraction ou répulsion selon le mode
  p.vx += dx / dist * f * direction;
  p.vy += dy / dist * f * direction;

  // Friction (amortissement)
  p.vx *= friction;
  p.vy *= friction;

  p.x += p.vx;
  p.y += p.vy;
});