Show track properties (distance, elevation gain, duration)

Maptool::TrackStats computes these once, shared by Gpx and Fit so
both track feature shapes carry the same properties regardless of
source format. Hovering a track line (not clicking — these describe
the whole line, not one point) shows them in a popup, same mechanism
GPX waypoints already use.

docs/architecture.adoc

@@ -233,6 +233,23 @@ assembling any HTML; the URL pattern excludes quote characters, so a
 matched URL can never break out of the `href="..."` attribute. Every
 other value is HTML-escaped before being handed to `Popup#setHTML`.
 
+Track properties:: Every track feature (both formats) also carries
+`distance_meters`, `elevation_gain_meters`, `duration_secs` and
+`point_count`, computed once server-side by `Maptool::TrackStats`
+(`lib/maptool/track_stats.rb`, shared by `Gpx`/`Fit` so neither
+duplicates the haversine formula) rather than in the browser — cheap
+either way, but it means a GPX and a FIT file end up with the same
+properties shape, so the frontend doesn't need to know which format it
+got. Distance sums the haversine distance between consecutive points;
+elevation gain sums only the positive deltas (descents don't
+subtract); duration is the gap between the first and last known
+timestamp, skipping unknown ones in between — `nil` for any of these
+where the source data doesn't have enough to compute it (e.g. a GPX
+file with no `<time>` elements has `duration_secs: nil`). Hovering a
+track line (rather than clicking, unlike a waypoint — these properties
+describe the whole line, not whichever point the cursor happens to be
+over) shows them in the same popup mechanism.
+
 == Live GNSS position
 
 A "GNSS" map control (`public/js/gnss.js`) shows the fix from a local