refactor(tracking): tighten sf_tracking package

- Lazy-init sfTracking to avoid touching Firebase at import time
- DRY SfTrackingRepository with a single _broadcast helper
- Drop empty DashboardTracking, fix double step_completed in device_setup
- Move yearsBetween to packages/utils
- Add 5 unit tests for SfTrackingRepository
- Strip noisy comments from mixins and view models

packages/utils/lib/src/date_utils.dart

@@ -0,0 +1,10 @@
+/// Returns the number of full years between [from] and [to], using the
+/// classic "have we already passed the birthday this year?" rule. Returns
+/// 0 if [from] is in the future relative to [to].
+int yearsBetween(DateTime from, DateTime to) {
+  var years = to.year - from.year;
+  final hasNotReachedAnniversary =
+      to.month < from.month || (to.month == from.month && to.day < from.day);
+  if (hasNotReachedAnniversary) years -= 1;
+  return years < 0 ? 0 : years;
+}

packages/utils/lib/utils.dart

@@ -1,2 +1,3 @@
+export 'src/date_utils.dart';
 export 'src/size_utils.dart';
 export 'src/test.dart';