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.

app.rb

@@ -62,6 +62,7 @@ module Maptool
 
     OFFLINE_PMTILES_PATH = File.join(DATA_DIR, "offline.pmtiles")
     TERRAIN_PMTILES_PATH = File.join(DATA_DIR, "terrain.pmtiles")
+    HIKING_TRAILS_PATH = PmtilesBuilder.hiking_trails_path(DATA_DIR)
     GPX_STORE = GpxStore.new(File.join(DATA_DIR, "gpx"))
     LIVETRACK_STORE = LiveTrackStore.new(File.join(DATA_DIR, "livetrack.json"))
 
@@ -356,6 +357,30 @@ module Maptool
       json ok: true
     end
 
+    # Generated as a step inside POST /api/offline-tiles/build (see
+    # PmtilesBuilder.extract_hiking_trails) rather than its own separate
+    # build — it's derived from the exact same per-country .osm.pbf cache
+    # offline.pmtiles itself already downloads, so there's nothing of its
+    # own to trigger or delete; DELETE /api/offline-tiles removes this
+    # file too. No `meta` argument here (unlike offline/terrain above) —
+    # parsing the whole GeoJSON just to report a feature count would cost
+    # real time on every page load's status check for what's a multi-MB
+    # file across several countries; size/mtime alone is enough to know
+    # whether it's there.
+    get "/api/hiking-trails" do
+      json pmtiles_file_status(HIKING_TRAILS_PATH, nil)
+    end
+
+    # Fetched client-side only when the Hiking trails overlay is actually
+    # toggled on (public/js/hiking_trails.js), not on page load — same
+    # "on demand" story as the online raster tiles it replaces once this
+    # is available.
+    get "/tiles/hiking_trails.geojson" do
+      halt 404 unless File.exist?(HIKING_TRAILS_PATH)
+
+      send_file HIKING_TRAILS_PATH, type: "application/geo+json"
+    end
+
     # Pinged once per page load (public/js/map.js) — url is a same-origin
     # path (not kiwix-serve's own address, which the browser can't reach
     # directly behind the reverse proxy in front of this app), proxied by