Show a GeoJSON icon next to files with a ready cache

GpxStore#tree now reports a "cached" flag per file (reusing the same
mtime freshness check geojson() already does); the file tree shows a
small curly-braces icon next to any file whose conversion is already
on disk, so it's visible which files will toggle on instantly versus
which still need to parse.

test/test_gpx_store.rb

@@ -121,6 +121,30 @@ class GpxStoreTest < Minitest::Test
     assert_raises(Errno::ENOENT) { @store.geojson("nope.gpx") }
   end
 
+  def test_tree_reports_cached_true_after_a_successful_save
+    @store.save(folder: "", filename: "ride.gpx", io: StringIO.new(sample_gpx))
+
+    file = @store.tree["children"].find { |c| c["name"] == "ride.gpx" }
+    assert file["cached"]
+  end
+
+  def test_tree_reports_cached_false_for_an_unparseable_file
+    @store.save(folder: "", filename: "bad.gpx", io: StringIO.new("not gpx at all"))
+
+    file = @store.tree["children"].find { |c| c["name"] == "bad.gpx" }
+    refute file["cached"]
+  end
+
+  def test_tree_reports_cached_false_once_the_source_is_resaved
+    path = @store.save(folder: "", filename: "ride.gpx", io: StringIO.new(sample_gpx))
+    FileUtils.touch(@store.resolve_file(path) + ".geojson", mtime: Time.now - 10)
+    File.write(@store.resolve_file(path), sample_gpx)
+    FileUtils.touch(@store.resolve_file(path), mtime: Time.now)
+
+    file = @store.tree["children"].find { |c| c["name"] == "ride.gpx" }
+    refute file["cached"]
+  end
+
   private
 
   def sample_gpx