Support uploading .geojson files directly

A .geojson file needs no GPX/FIT conversion — it's already the shape
gpx.js expects — so GpxStore#geojson now just reads and returns it
as-is, no cache sidecar. save/tree/the upload dropzone and its zip
handling all pick it up through the existing TRACK_EXTENSIONS list.

Also fixes a bug the new "cached" tree icon would have shipped with:
a GPX/FIT's own .geojson cache sidecar (e.g. "ride.gpx.geojson") also
matches the new .geojson branch in list_children, which would have
listed it as a second, phantom tree entry — excluded explicitly, with
a regression test.

test/test_gpx_store.rb

@@ -145,6 +145,35 @@ class GpxStoreTest < Minitest::Test
     refute file["cached"]
   end
 
+  def test_geojson_extension_is_preserved_not_forced_to_gpx
+    path = @store.save(folder: "", filename: "spots.geojson", io: StringIO.new(sample_geojson))
+    assert_equal "spots.geojson", path
+  end
+
+  def test_uploaded_geojson_is_served_as_is_with_no_cache_file
+    path = @store.save(folder: "", filename: "spots.geojson", io: StringIO.new(sample_geojson))
+
+    assert_equal sample_geojson, @store.geojson(path)
+    refute File.file?(@store.resolve_file(path) + ".geojson"), "a .geojson source needs no separate cache sidecar"
+  end
+
+  def test_uploaded_geojson_is_always_reported_cached
+    @store.save(folder: "", filename: "spots.geojson", io: StringIO.new(sample_geojson))
+
+    file = @store.tree["children"].find { |c| c["name"] == "spots.geojson" }
+    assert file["cached"]
+    assert_equal ["waypoint"], file["kinds"]
+  end
+
+  # A GPX/FIT's own GeoJSON cache sidecar (e.g. "ride.gpx.geojson") also
+  # ends in ".geojson" — regression test for it not showing up as its own
+  # second, phantom tree entry alongside "ride.gpx".
+  def test_geojson_cache_sidecar_is_not_listed_as_its_own_tree_entry
+    @store.save(folder: "", filename: "ride.gpx", io: StringIO.new(sample_gpx))
+
+    assert_equal ["ride.gpx"], @store.tree["children"].map { |c| c["name"] }
+  end
+
   private
 
   def sample_gpx
@@ -152,4 +181,8 @@ class GpxStoreTest < Minitest::Test
       <gpx><wpt lat="48.1" lon="17.1"><name>Waypoint</name></wpt></gpx>
     GPX
   end
+
+  def sample_geojson
+    '{"type":"FeatureCollection","features":[{"type":"Feature","properties":{"kind":"waypoint","name":"Spot"},"geometry":{"type":"Point","coordinates":[17.1,48.1]}}]}'
+  end
 end