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.

public/js/overlays.js

@@ -245,6 +245,10 @@ export class OverlaysControl {
     this._hillshade.setOfflineAvailable(available);
   }
 
+  setHikingTrailsOfflineAvailable(available) {
+    this._hikingTrails.setOfflineAvailable(available);
+  }
+
   // Called by map.js's PitchToggleControl whenever the 3D pitch toggle
   // changes, so raised terrain follows the camera tilt rather than needing
   // the "Terrain shading" checkbox re-clicked to notice.