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/src/app.dart

@@ -7,6 +7,8 @@ import 'design/zen_style.dart';
 import 'exercises/views/exercise_detail_view.dart';
 import 'home/home_view.dart';
 import 'localization/app_localizations.dart';
+import 'plans/views/exercise_picker_view.dart';
+import 'plans/views/plan_editor_view.dart';
 import 'settings/settings_controller.dart';
 
 class ZenXerciseApp extends StatelessWidget {
@@ -34,6 +36,8 @@ class ZenXerciseApp extends StatelessWidget {
         settings: settings,
         builder: (context) => switch (settings.name) {
           ExerciseDetailView.routeName => ExerciseDetailView(exerciseId: settings.arguments! as String),
+          PlanEditorView.routeName => PlanEditorView(planId: settings.arguments! as String),
+          ExercisePickerView.routeName => ExercisePickerView(planId: settings.arguments! as String),
           _ => const HomeView(),
         },
       ),