Split offline hiking trails per country, add a picker instead of one big file

data/hiking_trails.geojson is now data/hiking_trails/<CODE>.geojson —
one file per country instead of a merge, generated per-source
(PmtilesBuilder.extract_hiking_trails), cached the same way
geofabrik_cache is (skipped once a country's own extract is already
newer than its source .osm.pbf, so growing the selection by one
country only re-extracts that one). GET /api/hiking-trails now lists
available country codes instead of a single file's status.

The Overlays panel's "Hiking trails" item becomes an expandable
per-country checklist once any offline extract exists (same shape as
Radar's own expand), replacing the plain on/off checkbox — nobody
covering a dozen countries' worth of offline map data needs all of
their hiking trails loaded and parsed at once, so each ticked country
fetches its own GeoJSON independently instead.

HikingTrails.extract_merged (server-side multi-country merge, no
longer needed now that the client picks per country) removed again.
Verified the new extract_hiking_trails + caching skip live on brick
against a real cached country PBF.

TODO.adoc

@@ -13,8 +13,7 @@ independence from connectivity.
 ** [ ] 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 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] 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. One `data/hiking_trails/<CODE>.geojson` per country (not one combined file), generated as the last step of every offline build and cached like `geofabrik_cache` (skipped once already newer than its source PBF) — a many-country selection no longer means one giant file, since `GET /api/hiking-trails` lists available countries and the Overlays panel's own checklist only fetches `GET /tiles/hiking_trails/<CODE>.geojson` for whichever ones get ticked. Offline click uses a local `queryRenderedFeatures` hit-test instead of Overpass — instant, no network. See docs/development.adoc
 * [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