feat: move sensor details to sheet, implement placement on floor plan

lib/providers.dart

@@ -148,6 +148,10 @@ final sensorProvider =
 
 final selectedSensorIdProvider = StateProvider<String?>((ref) => null);
 
+/// Non-null while the user is placing a sensor on the floor plan via the
+/// center-dot placement flow. Cleared on Place, Cancel, or back navigation.
+final sensorPlacementProvider = StateProvider<Sensor?>((ref) => null);
+
 final floorPlanModeProvider =
     StateProvider<FloorPlanMode>((ref) => FloorPlanMode.view);