drawVertexDot function
Draws a small filled dot with a white halo — used to mark line vertices.
Implementation
void drawVertexDot(Canvas canvas, Offset center, Color color, double radius) {
canvas
..drawCircle(
center,
radius * 1.7,
Paint()..color = const Color(0xFFFFFFFF),
)
..drawCircle(center, radius, Paint()..color = color);
}