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/fixtures/sample.gpx

@@ -8,14 +8,27 @@
   </wpt>
   <trk>
     <name>Morning Loop</name>
+    <type>running</type>
     <trkseg>
       <trkpt lat="48.1" lon="17.2">
         <ele>200</ele>
         <time>2026-01-01T09:00:00Z</time>
+        <extensions>
+          <gpxtpx:TrackPointExtension xmlns:gpxtpx="http://www.garmin.com/xmlschemas/TrackPointExtension/v1">
+            <gpxtpx:hr>140</gpxtpx:hr>
+            <gpxtpx:cad>80</gpxtpx:cad>
+          </gpxtpx:TrackPointExtension>
+        </extensions>
       </trkpt>
       <trkpt lat="48.101" lon="17.201">
         <ele>250</ele>
         <time>2026-01-01T09:10:00Z</time>
+        <extensions>
+          <gpxtpx:TrackPointExtension xmlns:gpxtpx="http://www.garmin.com/xmlschemas/TrackPointExtension/v1">
+            <gpxtpx:hr>155</gpxtpx:hr>
+            <gpxtpx:cad>84</gpxtpx:cad>
+          </gpxtpx:TrackPointExtension>
+        </extensions>
       </trkpt>
       <trkpt lat="48.102" lon="17.202">
         <ele>230</ele>