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/js/overlays.js

@@ -219,7 +219,7 @@ export class OverlaysControl {
       <li class="overlays-panel__item overlays-panel__item--expandable"></li>
       <li class="overlays-panel__item overlays-panel__item--expandable"></li>
       <li class="overlays-panel__item"></li>
-      <li class="overlays-panel__item"></li>
+      <li class="overlays-panel__item overlays-panel__item--expandable"></li>
     `;
     const [hillshadeItem, wikiItem, radarItem, shadowItem, contourItem, hikingTrailsItem] = list.children;
     this._hillshade.mount(hillshadeItem);
@@ -245,8 +245,8 @@ export class OverlaysControl {
     this._hillshade.setOfflineAvailable(available);
   }
 
-  setHikingTrailsOfflineAvailable(available) {
-    this._hikingTrails.setOfflineAvailable(available);
+  setHikingTrailsOfflineCountries(countries) {
+    this._hikingTrails.setOfflineCountries(countries);
   }
 
   // Called by map.js's PitchToggleControl whenever the 3D pitch toggle