How to use

1. Draw — click to add points, double-click to close ground
2. Edge click — insert a point on an existing edge (on the line)
3. Start / Goal — place on the top edge of ground
4. Grid ON — snap to 10px grid (edges align at x=0 and x=360)
5. Play — drag to aim and release to shoot
6. Post — save and get share URL (always creates a new URL)