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.

lib/maptool/fit.rb

@@ -67,7 +67,7 @@ module Maptool
         "geometry" => { "type" => "MultiLineString", "coordinates" => [coordinates] },
         "properties" => { "kind" => "track", "name" => nil, "activity_type" => result.activity_type }
           .merge(TrackStats.compute(coordinates, times: result.records.map { |r| r[:time] }))
-          .merge("profile" => TrackStats.build_profile(coordinates, extras: extras)),
+          .merge("profile" => TrackStats.build_profile(coordinates, times: result.records.map { |r| r[:time] }, extras: extras)),
       }
     end
     private_class_method :track_feature

lib/maptool/gpx.rb

@@ -72,7 +72,7 @@ module Maptool
         "geometry" => { "type" => "MultiLineString", "coordinates" => lines },
         "properties" => { "kind" => "track", "name" => child_text(trk, "name"), "activity_type" => child_text(trk, "type") }
           .merge(TrackStats.compute(all_points, times: times))
-          .merge("profile" => TrackStats.build_profile(all_points, extras: extras)),
+          .merge("profile" => TrackStats.build_profile(all_points, times: times, extras: extras)),
       }
     end
     private_class_method :track_feature

lib/maptool/track_stats.rb

@@ -30,10 +30,13 @@ module Maptool
     # One entry per coordinate, for the profile chart: cumulative distance
     # (computed the same way for every format, rather than trusting a
     # device's own odometer field — keeps a GPX and a FIT file directly
-    # comparable), elevation, and whatever per-point extras (speed/cadence/
-    # heart rate) the caller has, nil where a format/file doesn't carry it.
+    # comparable), elapsed time since the first known timestamp, elevation,
+    # and whatever per-point extras (speed/cadence/heart rate) the caller
+    # has — nil wherever a format/file doesn't carry it.
     # `extras`: array of hashes parallel to `coordinates`, or shorter/empty.
-    def self.build_profile(coordinates, extras: [])
+    # `times`: array of Time (or nil) parallel to `coordinates`, or [].
+    def self.build_profile(coordinates, times: [], extras: [])
+      start_time = times.compact.first
       distance = 0.0
       previous = nil
 
@@ -41,9 +44,11 @@ module Maptool
         distance += haversine(previous[1], previous[0], lat, lon) if previous
         previous = [lon, lat]
         extra = extras[i] || {}
+        point_time = times[i]
 
         {
           "distance_meters" => distance.round(1),
+          "elapsed_secs" => point_time && start_time ? (point_time - start_time).to_i : nil,
           "elevation_meters" => ele,
           "speed_mps" => extra[:speed_mps],
           "cadence_rpm" => extra[:cadence_rpm],