Prove the hard part

This tests the three things that make repeatable tree-lean measurement possible in a plain web app: a ghost overlay to reshoot from the same spot, a gravity-driven plumb line for true vertical, and tap-to-trace lean angle.

1. Grant camera + motion access on the next tap.
2. Point at a tree (or any near-vertical edge — a doorframe works).
3. Shoot, then tap the trunk base and a point up the trunk.
4. Save it as a reference, move, and try to line the ghost back up.