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/ui/routes/route_planner_screen.dart

@@ -202,6 +202,8 @@ class _RoutePlannerScreenState extends ConsumerState<RoutePlannerScreen>
                               Polyline(
                                 points: [
                                   for (final w in waypoints) ll.LatLng(w.latitude, w.longitude),
+                                  if (route.isClosedLoop)
+                                    ll.LatLng(waypoints.first.latitude, waypoints.first.longitude),
                                 ],
                                 strokeWidth: 4,
                                 // Visibly distinct from a recorded path (see the
@@ -315,10 +317,14 @@ class _RoutePlannerScreenState extends ConsumerState<RoutePlannerScreen>
                     right: 8,
                     child: _OverflowMenu(
                       hasWaypoints: waypoints.isNotEmpty,
+                      canCloseLoop: waypoints.length >= 2,
+                      isClosedLoop: route.isClosedLoop,
                       onRename: () => _rename(context, repo, route),
                       onDownload: waypoints.isEmpty
                           ? null
                           : () => _downloadOfflineTiles(context, waypoints),
+                      onToggleLoop: () =>
+                          repo.setClosedLoop(widget.routeId, !route.isClosedLoop),
                       onDelete: () async {
                         await repo.deleteRoutePlan(widget.routeId);
                         widget.onBack?.call();
@@ -509,14 +515,20 @@ class _DropPinTooltipState extends State<_DropPinTooltip>
 class _OverflowMenu extends StatelessWidget {
   const _OverflowMenu({
     required this.hasWaypoints,
+    required this.canCloseLoop,
+    required this.isClosedLoop,
     required this.onRename,
     required this.onDownload,
+    required this.onToggleLoop,
     required this.onDelete,
   });
 
   final bool hasWaypoints;
+  final bool canCloseLoop;
+  final bool isClosedLoop;
   final VoidCallback onRename;
   final VoidCallback? onDownload;
+  final VoidCallback onToggleLoop;
   final VoidCallback onDelete;
 
   @override
@@ -528,6 +540,7 @@ class _OverflowMenu extends StatelessWidget {
       onSelected: (value) => switch (value) {
         'rename' => onRename(),
         'download' => onDownload?.call(),
+        'closeLoop' => onToggleLoop(),
         'delete' => onDelete(),
         _ => null,
       },
@@ -543,6 +556,12 @@ class _OverflowMenu extends StatelessWidget {
           enabled: hasWaypoints,
           child: const Text('Download offline tiles'),
         ),
+        PopupMenuItem(
+          key: const Key('toggle-closed-loop'),
+          value: 'closeLoop',
+          enabled: canCloseLoop,
+          child: Text(isClosedLoop ? 'Open the loop' : 'Close the loop'),
+        ),
         const PopupMenuItem(
           key: Key('delete-route'),
           value: 'delete',