feat: render particle clouds for selected tags

lib/providers.dart

@@ -190,7 +190,12 @@ final roomOccupancyProvider = StreamProvider<Map<int, List<String>>>((ref) {
   return ref.watch(tagRepositoryProvider).watchRoomOccupancy();
 });
 
-final particleCloudProvider = StreamProvider<List<Particle>>((ref) {
-  final repo = ref.watch(tagRepositoryProvider);
-  return repo.watchParticleCloud();
+/// The tag whose particle cloud is being visualised on the floor plan.
+/// Null when no tag is tracked.
+final trackedTagProvider = StateProvider<Tag?>((ref) => null);
+
+/// Live particle snapshots for a specific tag (keyed by tag_id string).
+final particleSnapshotProvider =
+    StreamProvider.autoDispose.family<ParticleSnapshot, String>((ref, tagId) {
+  return ref.watch(tagRepositoryProvider).watchParticleCloud(tagId);
 });