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/index.erb

@@ -17,6 +17,15 @@
   </div>
 </div>
 
+<div id="profile-panel" class="profile-panel" hidden>
+  <div class="profile-panel__header">
+    <strong class="profile-panel__title"></strong>
+    <span class="profile-panel__stats"></span>
+    <button type="button" class="profile-panel__close" aria-label="Close">&times;</button>
+  </div>
+  <div class="profile-panel__charts"></div>
+</div>
+
 <div id="basemap-panel" class="basemap-panel" hidden>
   <div class="basemap-panel__header">
     <strong>Base map</strong>