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/support/test_app.dart

@@ -6,8 +6,11 @@ import 'package:zenxercise/src/app.dart';
 import 'package:zenxercise/src/exercises/exercise.dart';
 import 'package:zenxercise/src/exercises/exercise_library.dart';
 import 'package:zenxercise/src/modes/mode.dart';
+import 'package:zenxercise/src/plans/plan_repository.dart';
 import 'package:zenxercise/src/settings/settings_controller.dart';
 
+import 'test_database.dart';
+
 Exercise exercise(
   String id,
   Mode mode, {
@@ -58,15 +61,19 @@ Future<SettingsController> pumpApp(
   WidgetTester tester, {
   ExerciseLibrary? library,
   Map<String, Object> prefs = const {},
+  PlanRepository? plans,
 }) async {
   SharedPreferences.setMockInitialValues(prefs);
   final settings = SettingsController(SettingsService());
   await settings.loadSettings();
+  final db = testDatabase();
+  addTearDown(db.close);
   await tester.pumpWidget(
     MultiProvider(
       providers: [
         ChangeNotifierProvider.value(value: settings),
         Provider.value(value: library ?? sampleLibrary),
+        Provider.value(value: plans ?? PlanRepository(db)),
       ],
       child: const ZenXerciseApp(),
     ),

test/support/test_database.dart

@@ -0,0 +1,21 @@
+import 'dart:ffi';
+import 'dart:io';
+
+import 'package:drift/drift.dart';
+import 'package:drift/native.dart';
+import 'package:sqlite3/open.dart';
+import 'package:zenxercise/src/database/database.dart';
+
+/// An in-memory database for tests. On Windows hosts the sqlite3 library comes
+/// from ZENXERCISE_SQLITE_DLL if set, otherwise from the system's winsqlite3.dll.
+AppDatabase testDatabase() {
+  if (Platform.isWindows) {
+    open.overrideFor(OperatingSystem.windows, () {
+      final custom = Platform.environment['ZENXERCISE_SQLITE_DLL'];
+      return DynamicLibrary.open(custom ?? 'winsqlite3.dll');
+    });
+  }
+  driftRuntimeOptions.dontWarnAboutMultipleDatabases = true;
+  // Synchronous stream teardown keeps widget tests from hanging on pending timers.
+  return AppDatabase(DatabaseConnection(NativeDatabase.memory(), closeStreamsSynchronously: true));
+}