Add shared UI components for mesh application

- Introduced `mesh_ui.dart` with reusable widgets including SectionHeader, MeshCard, StatusChip, StatTile, AvatarCircle, SignalBars, RouteChip, PulseDot, BottomSheetHeader, ErrorRetryCard, and ListEntrance.
- Implemented `path_map_ui.dart` for path map screens, featuring path distance calculations, playback controls, and a summary list of observed paths.
- Created `themed_map_tile_layer.dart` for shared cached map tiles with automatic dark-mode treatment.

lib/helpers/snack_bar_builder.dart

@@ -25,7 +25,19 @@ void showDismissibleSnackBar(
   DismissDirection? dismissDirection,
   Clip? clipBehavior,
 }) {
-  final messenger = ScaffoldMessenger.of(context);
+  // Callers often reach here after an async gap; the context may already be
+  // unmounted, or deactivated (popped but not yet disposed) — ancestor
+  // lookups on a deactivated element throw. Showing nothing is the right
+  // outcome in both cases.
+  if (!context.mounted) return;
+  var isActive = true;
+  assert(() {
+    isActive = (context as Element).debugIsActive;
+    return true;
+  }());
+  if (!isActive) return;
+  final messenger = ScaffoldMessenger.maybeOf(context);
+  if (messenger == null) return;
   messenger.showSnackBar(
     SnackBar(
       key: key,