init: rough companion app stub

lib/domain/models/onboarding_status.dart

@@ -0,0 +1,13 @@
+enum OnboardingStatus {
+  /// No admin user exists yet.
+  notStarted,
+
+  /// Admin user created, floor plan not yet saved.
+  awaitingFloorPlan,
+
+  /// Floor plan saved, no sensors enrolled yet.
+  awaitingFirstSensor,
+
+  /// At least one sensor enrolled; onboarding considered complete.
+  complete,
+}