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/ui/router.dart

@@ -21,50 +21,48 @@ abstract final class Routes {
 }
 
 GoRouter buildRouter() => GoRouter(
-      initialLocation: Routes.record,
-      routes: [
-        GoRoute(
-          path: Routes.record,
-          builder: (context, state) => RecordScreen(
-            onOpenTrips: () => context.push(Routes.trips),
-          ),
-        ),
-        GoRoute(
-          path: Routes.trips,
-          builder: (context, state) => TripsScreen(
-            onOpenTrip: (id) => context.push(Routes.detailFor(id)),
-            onBack: () => context.pop(),
-          ),
-        ),
-        GoRoute(
-          path: Routes.tripDetail,
-          builder: (context, state) {
-            // Parsed explicitly. A malformed or missing id must land on a real screen
-            // saying so, never on a blank one or a crash.
-            final id = int.tryParse(state.pathParameters['tripId'] ?? '');
-            if (id == null) {
-              return const _NotFound();
-            }
-            return TripDetailScreen(tripId: id, onBack: () => context.pop());
-          },
-        ),
-      ],
-      errorBuilder: (context, state) => const _NotFound(),
-    );
+  initialLocation: Routes.record,
+  routes: [
+    GoRoute(
+      path: Routes.record,
+      builder: (context, state) =>
+          RecordScreen(onOpenTrips: () => context.push(Routes.trips)),
+    ),
+    GoRoute(
+      path: Routes.trips,
+      builder: (context, state) => TripsScreen(
+        onOpenTrip: (id) => context.push(Routes.detailFor(id)),
+        onBack: () => context.pop(),
+      ),
+    ),
+    GoRoute(
+      path: Routes.tripDetail,
+      builder: (context, state) {
+        // Parsed explicitly. A malformed or missing id must land on a real screen
+        // saying so, never on a blank one or a crash.
+        final id = int.tryParse(state.pathParameters['tripId'] ?? '');
+        if (id == null) {
+          return const _NotFound();
+        }
+        return TripDetailScreen(tripId: id, onBack: () => context.pop());
+      },
+    ),
+  ],
+  errorBuilder: (context, state) => const _NotFound(),
+);
 
 class _NotFound extends StatelessWidget {
   const _NotFound();
 
   @override
   Widget build(BuildContext context) => Scaffold(
-        appBar: AppBar(
-          leading: IconButton(
-            icon: const Icon(Icons.arrow_back),
-            onPressed: () => context.canPop()
-                ? context.pop()
-                : context.go(Routes.record),
-          ),
-        ),
-        body: const Center(child: Text('That ride could not be found.')),
-      );
+    appBar: AppBar(
+      leading: IconButton(
+        icon: const Icon(Icons.arrow_back),
+        onPressed: () =>
+            context.canPop() ? context.pop() : context.go(Routes.record),
+      ),
+    ),
+    body: const Center(child: Text('That ride could not be found.')),
+  );
 }