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.

docs/development.adoc

@@ -106,11 +106,12 @@ size rather than assuming one, so this isn't a problem either way.
 The sidebar's "Hiking trails" overlay works online out of the box
 (waymarkedtrails.org raster tiles, `public/js/hiking_trails.js`) —
 clicking a trail fires a one-off Overpass query for its name/colour/
-network/distance. For it to work offline too, `data/hiking_trails.geojson`
-is generated automatically as the last step of every `offline.pmtiles`
-build (`Maptool::HikingTrails`, `lib/maptool/hiking_trails.rb`), straight
-from the *same* per-country `.osm.pbf` files already downloaded for
-`offline.pmtiles` itself — no separate download, no third-party service:
+network/distance. For it to work offline too, one `data/hiking_trails/
+<CODE>.geojson` per country is generated automatically as the last step
+of every `offline.pmtiles` build (`Maptool::HikingTrails`,
+`lib/maptool/hiking_trails.rb`), straight from the *same* per-country
+`.osm.pbf` files already downloaded for `offline.pmtiles` itself — no
+separate download, no third-party service:
 
 [source,shell]
 ----
@@ -130,20 +131,30 @@ tool stitches a route relation's member ways into one line for you
 waymarkedtrails.org and this app's own online Overpass click handler
 already do.
 
-Best-effort: a hiking-trails extraction problem on one country's PBF
-logs a warning and is skipped rather than failing the whole
-`offline.pmtiles` build over what's a bonus overlay, not the core map
-(`PmtilesBuilder.extract_hiking_trails`). Coordinates are rounded to 5
-decimal places (~1m, plenty for a rendered line) to keep the combined
-file smaller — for a many-country selection it can still be a sizeable
-GeoJSON (Slovakia alone: ~3000 routes, ~30MB unrounded), fetched
-client-side only once the overlay is actually toggled on, not on page
-load. `rake hiking:trails[pbf,out]` runs the extraction by hand against
-one `.osm.pbf` for testing.
+Best-effort per country: an extraction problem on one country's PBF logs
+a warning and is skipped rather than failing the whole `offline.pmtiles`
+build, or even the other countries in the same selection, over what's a
+bonus overlay, not the core map (`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
+region selection by one country only extracts that one. Coordinates are
+rounded to 5 decimal places (~1m, plenty for a rendered line) — Slovakia
+alone is ~3000 routes, ~30MB unrounded.
 
-`DELETE /api/offline-tiles` removes this file along with `offline.pmtiles`
-and the Geofabrik cache — there's no separate delete/rebuild route for it,
-since it's not an independent build.
+One file per country rather than one combined file: `GET /api/hiking-trails`
+lists which country codes have an extract (`countries: [...]`), and the
+Overlays panel's "Hiking trails" item turns that into a checklist —
+each ticked country fetches its own `GET /tiles/hiking_trails/<CODE>.geojson`
+only then, not on page load and not for countries never ticked. Offline
+click hit-tests the loaded vector layer(s) directly
+(`queryRenderedFeatures`) instead of querying Overpass, so it resolves
+instantly with no network at all. `rake hiking:trails[pbf,out]` runs the
+extraction by hand against one `.osm.pbf` for testing.
+
+`DELETE /api/offline-tiles` removes the whole `data/hiking_trails/`
+directory along with `offline.pmtiles` and the Geofabrik cache — there's
+no separate delete/rebuild route for it, since it's not an independent
+build.
 
 == In-app regeneration