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_track_stats.rb

@@ -32,4 +32,26 @@ class TrackStatsTest < Minitest::Test
     stats = Maptool::TrackStats.compute([[17.0, 48.0]])
     assert_equal 0.0, stats["distance_meters"]
   end
+
+  def test_build_profile_has_cumulative_distance_elevation_and_extras
+    points = [[17.0, 48.0, 100], [17.001, 48.0, 150]]
+    extras = [{ heart_rate_bpm: 120 }, { heart_rate_bpm: 130, cadence_rpm: 80 }]
+
+    profile = Maptool::TrackStats.build_profile(points, extras: extras)
+
+    assert_equal 2, profile.size
+    assert_equal 0.0, profile[0]["distance_meters"]
+    assert_equal 100, profile[0]["elevation_meters"]
+    assert_equal 120, profile[0]["heart_rate_bpm"]
+    assert_nil profile[0]["cadence_rpm"]
+    assert_operator profile[1]["distance_meters"], :>, 0
+    assert_equal 80, profile[1]["cadence_rpm"]
+    assert_equal [17.001, 48.0], [profile[1]["lon"], profile[1]["lat"]]
+  end
+
+  def test_build_profile_extras_default_to_nil_when_shorter_than_coordinates
+    profile = Maptool::TrackStats.build_profile([[17.0, 48.0], [17.001, 48.0]], extras: [])
+    assert_nil profile[0]["heart_rate_bpm"]
+    assert_nil profile[1]["speed_mps"]
+  end
 end