Legend items are now toggle buttons (icon grid, not a color-swatch
list) that hide/show that category's waypoints on the map. Filtering
re-slices each file's source data rather than using a layer filter,
since MapLibre clusters a GeoJSON source's full data regardless of
what a layer filter hides.
Captures ideas for offline maps/POI data, an NPS Unigrid-inspired
visual redesign, a 4-module menu reorg, and custom layer support.
Hardware-dependent features (leveling, Victron/HA telemetry) are
parked under Nice to have for later.
randy