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.

test/test_hiking_trails.rb

@@ -111,21 +111,6 @@ class HikingTrailsTest < Minitest::Test
     assert_equal [[[20.08765, 49.17912], [20.088, 49.18]]], coords
   end
 
-  def test_extract_merged_deduplicates_by_relation_id_across_files
-    feature = ->(id) {
-      { "type" => "Feature", "properties" => { "id" => id, "name" => "Trail #{id}" }, "geometry" => { "type" => "MultiLineString", "coordinates" => [] } }
-    }
-    # Relation 1 shows up in both "country" files (a cross-border trail);
-    # relation 2 only in the second.
-    per_file = { "at.osm.pbf" => [feature.call(1)], "cz.osm.pbf" => [feature.call(1), feature.call(2)] }
-    extract = ->(pbf_path) { { "type" => "FeatureCollection", "features" => per_file.fetch(pbf_path) } }
-
-    merged = Maptool::HikingTrails.extract_merged(%w[at.osm.pbf cz.osm.pbf], extract: extract)
-    ids = merged["features"].map { |f| f["properties"]["id"] }.sort
-
-    assert_equal [1, 2], ids
-  end
-
   def test_parse_opl_ignores_a_dangling_member_reference_but_keeps_the_rest
     opl = <<~OPL
       w1 Thighway=path Nn10x20.087y49.179,n11x20.088y49.18