Stage 3: Plan editor with Steps, Supersets and reordering

- Plan model with immutable edits: add with Mode defaults, move, remove,
  Superset with next, take a member out, ungroup; Stretching length
  estimate from holds, sides, rounds and Transitions
- drift database (plans table, items as JSON, ADR 0005) and
  PlanRepository; Plans list per Mode with rename/duplicate/delete
- Editor: drag handles, step editor sheet (sets, reps or time, optional
  kg; hold, both sides, rounds; Superset sets and rest between),
  pause between holds for Stretching Plans
- Exercise picker reusing the Library browser; Add to plan from the
  Exercise detail page
- Theme: Mode-styled snackbar, FAB, menus, segmented buttons, switches

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

test/plans/plan_repository_test.dart

@@ -0,0 +1,64 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:zenxercise/src/database/database.dart';
+import 'package:zenxercise/src/modes/mode.dart';
+import 'package:zenxercise/src/plans/models/plan.dart';
+import 'package:zenxercise/src/plans/plan_repository.dart';
+
+import '../support/test_database.dart';
+
+void main() {
+  late AppDatabase db;
+  late PlanRepository plans;
+  var now = DateTime(2026, 10, 1, 8);
+
+  setUp(() {
+    db = testDatabase();
+    now = DateTime(2026, 10, 1, 8);
+    plans = PlanRepository(db, clock: () => now);
+  });
+  tearDown(() => db.close());
+
+  test('a created Plan is empty, belongs to its Mode and is listed newest first', () async {
+    final morning = await plans.create(Mode.stretching, 'Morning');
+    now = now.add(const Duration(hours: 1));
+    final evening = await plans.create(Mode.stretching, 'Evening');
+    await plans.create(Mode.workout, 'Legs');
+
+    final listed = await plans.watchPlans(Mode.stretching).first;
+
+    expect(listed.map((p) => p.name), ['Evening', 'Morning']);
+    expect(morning.items, isEmpty);
+    expect(evening.transitionSeconds, Plan.defaultTransitionSeconds);
+  });
+
+  test('saving keeps the items and stamps the change time', () async {
+    final plan = await plans.create(Mode.workout, 'Push');
+    now = DateTime(2026, 10, 2);
+    await plans.save(plan.withExerciseAdded('Bench').withExerciseAdded('Dips').withSupersetWithNext(0));
+
+    final stored = (await plans.get(plan.id))!;
+
+    expect(stored.items.single, isA<Superset>());
+    expect(stored.updatedAt, DateTime(2026, 10, 2));
+    expect(stored.createdAt, plan.createdAt);
+  });
+
+  test('a duplicate copies the items under a new id and name', () async {
+    final plan = await plans.create(Mode.workout, 'Push');
+    await plans.save(plan.withExerciseAdded('Bench'));
+
+    final copy = await plans.duplicate(plan.id, 'Push (copy)');
+
+    expect(copy.id, isNot(plan.id));
+    expect(copy.items, (await plans.get(plan.id))!.items);
+    expect((await plans.watchPlans(Mode.workout).first), hasLength(2));
+  });
+
+  test('a deleted Plan is gone', () async {
+    final plan = await plans.create(Mode.workout, 'Push');
+
+    await plans.delete(plan.id);
+
+    expect(await plans.get(plan.id), isNull);
+  });
+}