FB-04: fix Route Planner map failing to render on cold navigation

routePlanProvider's AsyncLoading state collapsed with "route doesn't exist"
via .valueOrNull, so a brand-new route's screen briefly rendered "This route
no longer exists." with no FlutterMap in the tree before the DB stream's
first emission arrived. Add a hasValue guard mirroring waypointsAsync's
existing pattern, and replace the route planner's hardcoded initialZoom of
14 with FB-01's ambientZoom constant for street-level parity with the rest
of the app.

lib/src/ui/routes/route_planner_screen.dart

@@ -30,6 +30,7 @@ import '../components/glass_panel.dart';
 import '../components/ride_map.dart'
     show
         TileAttribution,
+        ambientZoom,
         maxTileZoom,
         tileMaxNativeZoom,
         tileSubdomains,
@@ -112,12 +113,27 @@ class _RoutePlannerScreenState extends ConsumerState<RoutePlannerScreen>
   @override
   Widget build(BuildContext context) {
     final colors = Theme.of(context).colorScheme;
-    final route = ref.watch(routePlanProvider(widget.routeId)).valueOrNull;
+    final routeAsync = ref.watch(routePlanProvider(widget.routeId));
     final waypointsAsync = ref.watch(routeWaypointsProvider(widget.routeId));
     final waypoints = waypointsAsync.valueOrNull ?? const [];
     final units = ref.watch(unitSystemProvider);
     final repo = ref.read(routePlanRepositoryProvider);
 
+    // Mirrors waypointsAsync's own guard below: `.valueOrNull` alone can't tell "still
+    // loading" apart from "genuinely doesn't exist" -- both collapse to null. On a
+    // brand-new route (fresh navigation right after `repo.createRoutePlan`), this
+    // provider's DB stream hasn't delivered its first row yet, so without this guard
+    // the screen would render "This route no longer exists." for a route that is
+    // completely valid, with zero FlutterMap in the tree, until that first emission
+    // arrives.
+    if (!routeAsync.hasValue) {
+      return const Scaffold(
+        backgroundColor: Colors.transparent,
+        body: Center(child: CircularProgressIndicator()),
+      );
+    }
+    final route = routeAsync.value;
+
     if (route == null) {
       return Scaffold(
         backgroundColor: Colors.transparent,
@@ -154,7 +170,7 @@ class _RoutePlannerScreenState extends ConsumerState<RoutePlannerScreen>
                         initialCenter: waypoints.isEmpty
                             ? const ll.LatLng(0, 0)
                             : ll.LatLng(waypoints.first.latitude, waypoints.first.longitude),
-                        initialZoom: waypoints.length <= 1 ? 14 : maxTileZoom - 3,
+                        initialZoom: waypoints.length <= 1 ? ambientZoom : maxTileZoom - 3,
                         maxZoom: maxTileZoom,
                         onTap: (tapPosition, point) {
                           repo.addWaypoint(widget.routeId, point.latitude, point.longitude);