Adds a «POI» checkbox to the terrain popup that toggles the
poi-circles and poi-labels layers via map.setLayoutProperty. The
choice is persisted in localStorage (key `poi-visible`) and restored
on page load and after style changes, kept consistent with the
runtime layerState.poi per ADR-0001.
Tests: behavioral JS unit tests (TP-01..TP-04) run via `node --test`,
wrapped by tests/unit/test_poi_toggle.py with static structure checks
so they execute under the existing `pytest tests/` CI step.
Refs: ET-002
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
