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/data/database.dart

@@ -121,6 +121,9 @@ class RoutePlans extends Table {
   /// Null in this ticket -- the polyline is derived from waypoints, not stored. V3-08
   /// fills it with the road-snapped geometry, which is not cheaply re-derivable.
   TextColumn get geometry => text().nullable()();
+
+  /// Whether the route loops back to its first waypoint after the last one. See FB-05.
+  BoolColumn get isClosedLoop => boolean().withDefault(const Constant(false))();
 }
 
 /// One pin on a [RoutePlans] row.
@@ -146,7 +149,7 @@ class AppDatabase extends _$AppDatabase {
   AppDatabase(super.e);
 
   @override
-  int get schemaVersion => 3;
+  int get schemaVersion => 4;
 
   @override
   MigrationStrategy get migration => MigrationStrategy(
@@ -162,6 +165,13 @@ class AppDatabase extends _$AppDatabase {
         await m.createTable(routePlans);
         await m.createTable(waypoints);
       }
+      // FB-05: closed-loop toggle, defaults to false for every pre-existing route.
+      // Guarded to `from >= 3` because a v2 -> v4+ upgrade just created `routePlans`
+      // fresh (above) with every current column, `isClosedLoop` included -- adding it
+      // again here would be a duplicate-column error.
+      if (from >= 3 && from < 4) {
+        await m.addColumn(routePlans, routePlans.isClosedLoop);
+      }
     },
     beforeOpen: (details) async {
       // Non-negotiable: without this the CASCADE relationships above do nothing.
@@ -553,6 +563,7 @@ class AppDatabase extends _$AppDatabase {
       distanceM: Value(route.distanceM),
       estimatedMillis: Value(route.estimatedMillis),
       geometry: Value(route.geometry),
+      isClosedLoop: Value(route.isClosedLoop),
     ),
   );
 
@@ -566,6 +577,12 @@ class AppDatabase extends _$AppDatabase {
     RoutePlansCompanion(distanceM: Value(distanceM)),
   );
 
+  Future<void> setRoutePlanClosedLoop(int id, bool value) => (update(
+    routePlans,
+  )..where((r) => r.id.equals(id))).write(
+    RoutePlansCompanion(isClosedLoop: Value(value)),
+  );
+
   /// Waypoints cascade with it.
   Future<void> deleteRoutePlan(int id) =>
       (delete(routePlans)..where((r) => r.id.equals(id))).go();
@@ -617,6 +634,7 @@ domain.RoutePlan _toRoutePlan(RoutePlanRow r) => domain.RoutePlan(
   distanceM: r.distanceM,
   estimatedMillis: r.estimatedMillis,
   geometry: r.geometry,
+  isClosedLoop: r.isClosedLoop,
 );
 
 domain.Waypoint _toWaypoint(WaypointRow r) => domain.Waypoint(