feat: render particle clouds for selected tags

lib/features/floorplan/floor_plan_screen.dart

@@ -4,6 +4,7 @@ import 'package:go_router/go_router.dart';
 
 import '../../domain/models/floor_plan_mode.dart';
 import '../../domain/models/sensor.dart';
+import '../../domain/models/tag.dart';
 import '../../providers.dart';
 import '../ble_provision/ble_provision_sheet.dart';
 import '../sensors/sensor_detail_sheet.dart';
@@ -87,6 +88,7 @@ class _FloorPlanScreenState extends ConsumerState<FloorPlanScreen> {
     final roomsAsync = ref.watch(roomsProvider);
     final sensorsAsync = ref.watch(sensorsProvider);
     final placingSensor = ref.watch(sensorPlacementProvider);
+    final trackedTag = ref.watch(trackedTagProvider);
 
     ref.listen(roomsProvider, (_, next) {
       next.whenData((r) => _editorKey.currentState?.loadFloorPlan(r));
@@ -112,8 +114,14 @@ class _FloorPlanScreenState extends ConsumerState<FloorPlanScreen> {
     ref.listen(tagPositionsProvider, (_, next) {
       next.whenData((t) => _editorKey.currentState?.updateTags(t));
     });
-    ref.listen(particleCloudProvider, (_, next) {
-      next.whenData((p) => _editorKey.currentState?.updateParticleCloud(p));
+    if (trackedTag != null) {
+      ref.listen(particleSnapshotProvider(trackedTag.tagId), (_, next) {
+        next.whenData(
+            (snap) => _editorKey.currentState?.updateParticleCloud(snap.particles));
+      });
+    }
+    ref.listen(trackedTagProvider, (prev, next) {
+      if (next == null) _editorKey.currentState?.updateParticleCloud([]);
     });
     ref.listen(sensorPlacementProvider, (prev, next) => _applyPlacementState());
 
@@ -244,6 +252,19 @@ class _FloorPlanScreenState extends ConsumerState<FloorPlanScreen> {
                         ],
                       ),
                     ),
+                  // Tracking card overlay
+                  if (trackedTag != null && placingSensor == null)
+                    Positioned(
+                      top: 16,
+                      left: 16,
+                      right: 16,
+                      child: _TrackingCard(
+                        tag: trackedTag,
+                        onStop: () {
+                          ref.read(trackedTagProvider.notifier).state = null;
+                        },
+                      ),
+                    ),
                   // Placement mode overlay
                   if (placingSensor != null) ...[
                     const Center(child: _PlacementDot()),
@@ -390,6 +411,42 @@ class _PlacementDot extends StatelessWidget {
   }
 }
 
+class _TrackingCard extends StatelessWidget {
+  const _TrackingCard({required this.tag, required this.onStop});
+
+  final Tag tag;
+  final VoidCallback onStop;
+
+  @override
+  Widget build(BuildContext context) {
+    final theme = Theme.of(context);
+    return Card(
+      elevation: 4,
+      child: Padding(
+        padding: const EdgeInsets.symmetric(horizontal: 12, vertical: 10),
+        child: Row(
+          children: [
+            const Icon(Icons.radar),
+            const SizedBox(width: 10),
+            Expanded(
+              child: Column(
+                crossAxisAlignment: CrossAxisAlignment.start,
+                mainAxisSize: MainAxisSize.min,
+                children: [
+                  Text(tag.name, style: theme.textTheme.labelLarge),
+                  Text('Tracking particle cloud',
+                      style: theme.textTheme.bodySmall),
+                ],
+              ),
+            ),
+            TextButton(onPressed: onStop, child: const Text('Stop')),
+          ],
+        ),
+      ),
+    );
+  }
+}
+
 class _PlacementCard extends StatelessWidget {
   const _PlacementCard({
     required this.sensor,

lib/features/floorplan/widgets/floor_plan_editor.dart

@@ -181,6 +181,7 @@ class FloorPlanEditorState extends State<FloorPlanEditor> {
       final payload = jsonEncode(
         particles.map((p) => {'x': p.x, 'y': p.y, 'weight': p.weight}).toList(),
       );
+
       await _controller.runJavaScript('window.companion.updateCloud($payload)');
     });
   }