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.
Route plans are their own entity (RoutePlan/Waypoint, schema v3), never joined into ride totals or the rides list. RoutePlanRepository recomputes straight-line distance after every waypoint mutation. RoutePlannerScreen supports tap-to-add, tap-to-delete, and hand-rolled drag-to-move via MapCamera's screen/latlng conversions.
Fixed a real bug found by testing: FlutterMap's initialCenter/initialZoom are read once at construction, so building the map before the waypoints stream's first emission froze the camera on null-island. Also documents a genuine ten-minute test hang traced to Stream.first on a Drift watch() query inside testWidgets, which needs a pump-driven Timer that flutter_test's fake zone never advances on its own.
