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_pmtiles_builder.rb

@@ -216,20 +216,44 @@ class PmtilesBuilderTest < Minitest::Test
     )
   end
 
-  def test_build_offline_also_writes_a_hiking_trails_geojson
+  def test_build_offline_also_writes_a_per_country_hiking_trails_geojson
     select_regions("SK")
     thread = Maptool::PmtilesBuilder.build_offline(data_dir: @data_dir, download_extract: fake_download)
     thread.join
 
     # The fake osmium binary doesn't produce real OPL output, just
-    # confirms the file lands and is valid (empty) GeoJSON — the real
-    # parsing logic is covered by test_hiking_trails.rb against real OPL
-    # fixtures.
-    path = File.join(@data_dir, "hiking_trails.geojson")
-    assert_path_exists path
+    # confirms the file lands (one per country) and is valid (empty)
+    # GeoJSON — the real parsing logic is covered by test_hiking_trails.rb
+    # against real OPL fixtures.
+    assert_equal ["SK"], Maptool::PmtilesBuilder.hiking_trails_countries(@data_dir)
+    path = Maptool::PmtilesBuilder.hiking_trails_country_path(@data_dir, "SK")
     assert_equal({ "type" => "FeatureCollection", "features" => [] }, JSON.parse(File.read(path)))
   end
 
+  def test_build_offline_only_re_extracts_hiking_trails_for_a_newly_added_country
+    select_regions("SK")
+    Maptool::PmtilesBuilder.build_offline(data_dir: @data_dir, download_extract: fake_download).join
+    File.write(@log, "")
+
+    select_regions("SK", "AT")
+    Maptool::PmtilesBuilder.build_offline(
+      data_dir: @data_dir,
+      download_extract: fake_download,
+      fetch_last_modified: ->(_url) { "Wed, 10 Sep 2026 08:00:01 GMT" },
+    ).join
+
+    # SK's cached extract is already newer than SK.osm.pbf (untouched
+    # since the first build), so only AT's hiking trails get (re-)extracted
+    # — SK.osm.pbf itself is also skipped by cached_extract's own
+    # Last-Modified check, "osmium merge" runs regardless since two
+    # countries are now selected.
+    assert_equal(
+      ["osmium merge", "planetiler --osm-path", "osmium tags-filter", "osmium add-locations-to-ways", "osmium cat"],
+      log_lines,
+    )
+    assert_equal %w[AT SK], Maptool::PmtilesBuilder.hiking_trails_countries(@data_dir)
+  end
+
   # The actual reasoning: hiking trails are a bonus overlay bolted onto the
   # existing offline.pmtiles build, not the core map — an osmium hiccup on
   # this one step shouldn't take down an otherwise-successful build.