Wire offline hiking trails into the real build, API, and renderer

data/hiking_trails.geojson now generates automatically as the last
step of every offline.pmtiles build (PmtilesBuilder.extract_hiking_trails,
best-effort — a failure here doesn't fail the whole build), merged
across all selected countries with cross-border dedup by OSM relation
id (HikingTrails.extract_merged). Served at GET /tiles/hiking_trails.geojson
and GET /api/hiking-trails, fetched client-side only once the overlay
is actually toggled on.

hiking_trails.js now prefers this local file whenever it exists,
regardless of which base map (online/offline) is active — same
"prefer local when available" story as HillshadeControl already uses
for terrain. Offline click hit-tests the real vector layer directly
(queryRenderedFeatures) instead of querying Overpass — instant, no
network. Coordinates are rounded to 5dp to keep the combined file
smaller; colour tag values are normalized against a known-safe set
before ever reaching a style paint property.

Verified extract_merged (new: combines multiple countries' extracts,
dedupes overlapping relations) against two real cached country PBFs
on brick directly, not just fixtures.

TODO.adoc

@@ -12,8 +12,9 @@ independence from connectivity.
 * [x] Shaded relief / DEM layer — hillshade toggle in the sidebar, sourced from public Terrarium tiles online; reads `data/terrain.pmtiles` for offline if one's been generated (see docs/development.adoc), otherwise the toggle is online-only for now
 ** [ ] Terrain slope readout for parking (numeric, not just visual shading)
 * [ ] Contour lines from OSM for hikes and mountain passes
-* [x] Hiking trail overlay — Waymarked Trails raster tiles (`public/js/hiking_trails.js`), colored per-route by the OSM `colour` tag they already render (the KST/KČT paint-blaze colors in Slovakia/Czechia, plain red elsewhere), so no styling logic needed here. Click queries a one-off Overpass `around` search (racing 3 public mirrors) for the route's name/colour/network/operator/distance, no server involved
-** [x] Prototype: an offline extraction path exists after all, no custom Planetiler profile needed — `Maptool::HikingTrails.extract` (`lib/maptool/hiking_trails.rb`, `rake hiking:trails[pbf,out]`) runs `osmium tags-filter r/route=hiking` + `osmium cat -f opl` against the *same* Geofabrik `.osm.pbf` already downloaded for `offline.pmtiles` — no new download, no new dependency. Not wired into the actual offline build/rendering yet, still deferred until there's a route planner to pair it with (see the entry above)
+* [x] Hiking trail overlay — Waymarked Trails raster tiles online (`public/js/hiking_trails.js`), colored per-route by the OSM `colour` tag they already render (the KST/KČT paint-blaze colors in Slovakia/Czechia, plain red elsewhere). Click queries a one-off Overpass `around` search (racing 3 public mirrors) for the route's name/colour/network/operator/distance
+** [x] Offline copy wired in for real, no custom Planetiler profile needed — `Maptool::HikingTrails` (`lib/maptool/hiking_trails.rb`) runs `osmium tags-filter r/route=hiking` + `add-locations-to-ways` + `cat -f opl` against the *same* per-country Geofabrik `.osm.pbf` already downloaded for `offline.pmtiles`, no new download. Generated automatically as the last step of every offline build (`PmtilesBuilder.extract_hiking_trails`, best-effort — a failure here doesn't fail the whole build) into `data/hiking_trails.geojson`, served at `GET /tiles/hiking_trails.geojson` only once the overlay's actually toggled on. Offline click uses a local `queryRenderedFeatures` hit-test instead of Overpass — instant, no network. See docs/development.adoc
+*** [ ] File size for a many-country selection could get large (Slovakia alone: ~3000 routes) — coordinates are rounded to 5dp but there's no real tiling/simplification yet; revisit if a big selection makes the browser fetch noticeably slow
 * [x] Overpass/OSM scripts to extract POIs (drinking water, dump stations, LPG, public toilets, laundromats, shops, springs) — `rake overpass:pois`, see docs/development.adoc; writes a `.geojson` to drop in `data/gpx/` or upload. Also runnable from inside the app itself now, see the import plugins entry below (`plugins/overpass-pois` wraps this same query). Each category now carries a real `POI_ICONS` glyph and color (not p4n2gpx's plain red marker) plus a popup description built from opening_hours/operator/website tags
 * [x] Import plugins: a pluggable way to add more data sources (Park4Night, iOverlander, Freecampsites, ...) without their scrapers necessarily living in this (public) repo — sidebar icon (logged in only) lists whatever's under `plugins/`, "Run for this area" uses the current map viewport as the bbox, no separate area-picker needed. Contract: a `plugin.json` manifest + a script that reads `--south`/`--west`/`--north`/`--east` and prints a GeoJSON `FeatureCollection` to stdout (`lib/maptool/plugins.rb`, `GET /api/plugins`, `POST /api/plugins/:name/run`) — output saved the same way an upload would be. `plugins/overpass-pois` ships as the reference example; a private/ToS-sensitive scraper just drops its own directory in locally and is never `git add`ed
 ** [x] Park4Night — private plugin (`data/plugins/p4n2gpx/`, outside this repo per the "ToS-sensitive scraper" note above); iOverlander/Freecampsites still open, same story