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.

public/css/style.css

@@ -1579,6 +1579,22 @@ html, body {
   font-size: 12px;
 }
 
+.hiking-trail-countries {
+  display: flex;
+  flex-wrap: wrap;
+  gap: 4px 12px;
+  list-style: none;
+  margin: 0;
+  padding: 0;
+}
+
+.hiking-trail-countries__item label {
+  display: flex;
+  align-items: center;
+  gap: 4px;
+  cursor: pointer;
+}
+
 .livetrack-panel__list {
   flex: 1;
   overflow-y: auto;