Turn the GPX category legend into a clickable Unigrid filter matrix

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.

TODO.adoc

@@ -22,7 +22,7 @@ independence from connectivity.
 * [ ] Restyle popovers, side panels, and point detail views after the NPS Unigrid layout system (Massimo Vignelli) with Frutiger/Rawlinson Roadside-style typography
 ** [ ] Top black banner (full width): NPS-style logo + "MAPTOOL" on the left, location telemetry (DMS/DD coordinates, DEM elevation) in the center, hamburger menu + offline-data status on the right, white sans-serif type
 ** [ ] Point detail/sidepanel: black header banner with white location name and an NPS pictogram in the corner; amenities (water, waste, power, slope) shown as a pictogram grid, no extra text; action buttons for "route here" (Valhalla), "start leveling", "save offline"
-** [ ] Filter panel as a "Unigrid Matrix" — an icon grid to toggle categories on/off, styled like the legend on an official NPS paper map, instead of plain dropdowns
+** [x] Filter panel as a "Unigrid Matrix" — an icon grid to toggle categories on/off, styled like the legend on an official NPS paper map, instead of plain dropdowns
 * [ ] Custom vector style (MVT/MapLibre) using the NPS color palette — muted natural tones for forests, water, and contours — so POIs and routes stand out at high zoom
 ** [ ] Landuse & protections: distinct styling for national parks, nature reserves (translucent green/gray-green), military zones, and private land (important for wild camping)
 ** [ ] Elevation-tinted terrain combined with offline hillshade for a characteristic relief look