Add activity type and a click-to-open track profile panel

Track properties gain activity_type (FIT: sport enum from the
session/sport message, Garmin's own table; GPX: <trk><type> passed
through as-is) alongside the existing distance/elevation/duration.

Clicking a track line now opens a bottom panel with a chart per
metric the track actually has (elevation always; speed/cadence/heart
rate only when present — FIT's record fields or GPX's device
extensions). TrackStats.build_profile computes cumulative distance
the same way for both formats so they're directly comparable. One
pointermove listener drives a synchronized crosshair across every
chart, a single tooltip listing all series, and a map marker at the
hovered point's position — hovering the chart shows where on the
track that moment was.

test/test_gpx.rb

@@ -23,11 +23,24 @@ class GpxTest < Minitest::Test
     assert_equal "MultiLineString", track["geometry"]["type"]
     assert_equal 3, track["geometry"]["coordinates"][0].size
     assert_equal "Morning Loop", track["properties"]["name"]
+    assert_equal "running", track["properties"]["activity_type"]
     assert_operator track["properties"]["distance_meters"], :>, 0
     assert_equal 50.0, track["properties"]["elevation_gain_meters"], "200->250 is the only gain, 250->230 descent not subtracted"
     assert_equal 1200, track["properties"]["duration_secs"]
   end
 
+  def test_track_profile_carries_extension_fields_and_tolerates_points_without_them
+    geojson = Maptool::Gpx.to_geojson(fixture_path("sample.gpx"))
+    profile = geojson["features"].find { |f| f["properties"]["kind"] == "track" }["properties"]["profile"]
+
+    assert_equal 3, profile.size
+    assert_equal 140, profile[0]["heart_rate_bpm"]
+    assert_equal 84, profile[1]["cadence_rpm"]
+    assert_nil profile[2]["heart_rate_bpm"], "third point has no <extensions>"
+    assert_equal 0.0, profile[0]["distance_meters"]
+    assert_equal [17.2, 48.1], [profile[0]["lon"], profile[0]["lat"]]
+  end
+
   def test_invalid_xml_raises_parse_error
     path = File.join(Dir.mktmpdir, "bad.gpx")
     File.write(path, "<gpx><trk unclosed>")