Add a Distance/Time toggle to the profile chart's x-axis

TrackStats.build_profile now also computes elapsed_secs per point
(relative to the first known timestamp), alongside the existing
cumulative distance. Point x-positions stay distance-based either
way — only the shared axis strip's 5 tick labels (and the tooltip)
switch between the two, so toggling is a cheap re-render rather than
rebuilding the charts. The toggle only appears when a track actually
has elapsed-time data to switch to.

test/test_track_stats.rb

@@ -54,4 +54,20 @@ class TrackStatsTest < Minitest::Test
     assert_nil profile[0]["heart_rate_bpm"]
     assert_nil profile[1]["speed_mps"]
   end
+
+  def test_build_profile_elapsed_secs_is_relative_to_the_first_known_timestamp
+    points = [[17.0, 48.0], [17.001, 48.0], [17.002, 48.0]]
+    times = [nil, Time.utc(2026, 1, 1, 0, 0, 0), Time.utc(2026, 1, 1, 0, 5, 0)]
+
+    profile = Maptool::TrackStats.build_profile(points, times: times)
+
+    assert_nil profile[0]["elapsed_secs"], "no timestamp yet at the first point"
+    assert_equal 0, profile[1]["elapsed_secs"], "first known timestamp is the zero point"
+    assert_equal 300, profile[2]["elapsed_secs"]
+  end
+
+  def test_build_profile_elapsed_secs_is_nil_without_any_times
+    profile = Maptool::TrackStats.build_profile([[17.0, 48.0]], times: [])
+    assert_nil profile[0]["elapsed_secs"]
+  end
 end