FB-05: closed-loop routes in Route Planner

Add an isClosedLoop toggle to route plans (schema v3 -> v4 migration),
threaded through the domain model, repository distance recomputation,
the overflow menu, and the drawn polyline, so a route can loop back to
its first waypoint.

lib/src/domain/models.dart

@@ -241,6 +241,7 @@ class RoutePlan {
     this.distanceM = 0.0,
     this.estimatedMillis,
     this.geometry,
+    this.isClosedLoop = false,
   });
 
   final int id;
@@ -256,6 +257,9 @@ class RoutePlan {
   /// never stored -- it is fully derived from them, so there is nothing to persist.
   final String? geometry;
 
+  /// Whether the route loops back to its first waypoint after the last one.
+  final bool isClosedLoop;
+
   RoutePlan copyWith({
     int? id,
     String? name,
@@ -264,6 +268,7 @@ class RoutePlan {
     double? distanceM,
     int? estimatedMillis,
     String? geometry,
+    bool? isClosedLoop,
   }) => RoutePlan(
     id: id ?? this.id,
     name: name ?? this.name,
@@ -272,6 +277,7 @@ class RoutePlan {
     distanceM: distanceM ?? this.distanceM,
     estimatedMillis: estimatedMillis ?? this.estimatedMillis,
     geometry: geometry ?? this.geometry,
+    isClosedLoop: isClosedLoop ?? this.isClosedLoop,
   );
 }