AR support for high-precision cone placement

Follow-up to the MVP (#1). GPS alone is limited to roughly 3–5 m accuracy; AR can do much better over short distances.

## Feature

Use ARKit to improve cone-placement precision beyond what GPS offers:

- **Scan the field**: use AR world tracking (and geo-tracking / `ARGeoTrackingConfiguration` where available) to build a locally accurate frame of reference.
- **Illustrate the exact spot**: render a marker in the camera view showing exactly where the cone should go on the ground.
- **Validate placement**: after the user drops a cone, detect/confirm its position and report the error from the ideal location.

## Notes

- `ARGeoTrackingConfiguration` availability varies by region/device; plan a fallback that anchors the AR session to one or two GPS-established reference points and uses relative AR distances (which are centimeter-accurate) from there.
- Cone-to-cone AR navigation could also replace long GPS walks with drift-corrected guidance.

Start by reviewing the MVP in issue #1 and the iOS project's existing location flow, then read Apple's ARKit documentation for ARGeoTrackingConfiguration and world tracking. Define how GPS-established reference points and the regional/device fallback should work before implementing. Done means the app can show the target spot, validate a dropped cone, and report placement error with an appropriate fallback.

Written by the indexing model from the issue text.