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>

lib/main.dart

@@ -1,15 +1,19 @@
+import 'package:drift_flutter/drift_flutter.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter/services.dart';
 import 'package:provider/provider.dart';
 
 import 'src/app.dart';
+import 'src/database/database.dart';
 import 'src/exercises/exercise_library.dart';
+import 'src/plans/plan_repository.dart';
 import 'src/settings/settings_controller.dart';
 
 void main() async {
   WidgetsFlutterBinding.ensureInitialized();
   await SystemChrome.setPreferredOrientations([DeviceOrientation.portraitUp, DeviceOrientation.portraitDown]);
 
+  final db = AppDatabase(driftDatabase(name: 'zenxercise'));
   final library = await ExerciseLibrary.load(rootBundle);
   final settings = SettingsController(SettingsService());
   await settings.loadSettings();
@@ -19,6 +23,7 @@ void main() async {
       providers: [
         ChangeNotifierProvider.value(value: settings),
         Provider.value(value: library),
+        Provider.value(value: PlanRepository(db)),
       ],
       child: const ZenXerciseApp(),
     ),