T19/T21: map and export, completing Phase 4

flutter_map with the same OSM tiles and no-API-key reasoning that chose osmdroid.
All four load-bearing behaviours ported and, unlike the native app's map, tested:
one polyline per segment so a pause is a real gap, render-only decimation, the
zoom clamp at OSM's max tile zoom with a short-ride fallback, and a real user
agent. Speed colouring is bucketed per run rather than per-vertex, since neither
osmdroid nor flutter_map makes per-vertex paint reasonable.

Export via share_plus, which also handles the iPad popover anchor a naive port
forgets. It passes the raw stored points, never the map's decimated path.

164 tests passing, analyze clean.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

lib/src/domain/models.dart

@@ -74,19 +74,18 @@ class Trip {
     double? maxSpeedKmh,
     double? elevationGainM,
     int? pointCount,
-  }) =>
-      Trip(
-        id: id ?? this.id,
-        startedAt: startedAt ?? this.startedAt,
-        endedAt: endedAt ?? this.endedAt,
-        name: name ?? this.name,
-        state: state ?? this.state,
-        distanceM: distanceM ?? this.distanceM,
-        movingMillis: movingMillis ?? this.movingMillis,
-        maxSpeedKmh: maxSpeedKmh ?? this.maxSpeedKmh,
-        elevationGainM: elevationGainM ?? this.elevationGainM,
-        pointCount: pointCount ?? this.pointCount,
-      );
+  }) => Trip(
+    id: id ?? this.id,
+    startedAt: startedAt ?? this.startedAt,
+    endedAt: endedAt ?? this.endedAt,
+    name: name ?? this.name,
+    state: state ?? this.state,
+    distanceM: distanceM ?? this.distanceM,
+    movingMillis: movingMillis ?? this.movingMillis,
+    maxSpeedKmh: maxSpeedKmh ?? this.maxSpeedKmh,
+    elevationGainM: elevationGainM ?? this.elevationGainM,
+    pointCount: pointCount ?? this.pointCount,
+  );
 }
 
 /// One pause-free stretch of recording within a [Trip].