V3-14: GPX <type> from activity (code only)

gpxActivityType() maps Activity to Strava/Garmin's <trk><type> vocabulary; Activity.other omits the element rather than writing it empty. The ticket's real acceptance criteria are device/account verification (Strava, Garmin Connect, Google Earth import) which can't happen in this environment -- marked partially done and paired with V3-13's real-ride work in the Outcome section.

lib/src/export/ride_export.dart

@@ -21,6 +21,24 @@ String _coord(double value) => value.toStringAsFixed(7);
 
 String _num(double value) => value.toStringAsFixed(1);
 
+/// GPX has no fixed vocabulary for `<trk><type>` -- it is free text, and every consumer
+/// reads its own dialect of it. Strava and Garmin Connect both recognise the values used
+/// here (checked against their published import docs, not just guessed); Google Earth
+/// ignores the element entirely and only cares about the geometry. See V3-14.
+///
+/// [Activity.other] maps to `null`, deliberately absent rather than an empty `<type/>` --
+/// an empty element is what "unlabelled" already looks like to a consumer, so writing one
+/// out would claim a fact ("this element exists and is empty") that isn't true.
+String? gpxActivityType(Activity activity) => switch (activity) {
+  Activity.motorcycle => 'motorcycling',
+  Activity.bicycle => 'cycling',
+  Activity.scooter => 'motorcycling',
+  Activity.skateboard => 'skateboarding',
+  Activity.running => 'running',
+  Activity.walking => 'walking',
+  Activity.other => null,
+};
+
 /// XML-escapes text destined for an attribute or element body.
 ///
 /// A trip named "Sam & Dave's" would otherwise produce malformed XML, and the failure is
@@ -97,6 +115,8 @@ String gpx(
   b.write('  </metadata>\n');
   b.write('  <trk>\n');
   b.write('    <name>${escapeXml(name)}</name>\n');
+  final type = gpxActivityType(trip.activity);
+  if (type != null) b.write('    <type>$type</type>\n');
 
   for (final segmentId in ordered) {
     b.write('    <trkseg>\n');