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.

views/layout.erb

@@ -25,7 +25,8 @@
         "/js/icons.js": "/js/icons.js?v=<%= ASSET_VERSION %>",
         "/js/livetrack.js": "/js/livetrack.js?v=<%= ASSET_VERSION %>",
         "/js/panels.js": "/js/panels.js?v=<%= ASSET_VERSION %>",
-        "/js/radar.js": "/js/radar.js?v=<%= ASSET_VERSION %>"
+        "/js/radar.js": "/js/radar.js?v=<%= ASSET_VERSION %>",
+        "/js/track_profile.js": "/js/track_profile.js?v=<%= ASSET_VERSION %>"
       }
     }
   </script>