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>

pubspec.yaml

@@ -7,6 +7,8 @@ environment:
   sdk: ^3.8.1
 
 dependencies:
+  drift: ^2.31.0
+  drift_flutter: ^0.2.8
   flutter:
     sdk: flutter
   flutter_localizations:
@@ -14,11 +16,15 @@ dependencies:
   intl: ^0.20.2
   provider: ^6.1.5+1
   shared_preferences: ^2.5.3
+  uuid: ^4.6.0
 
 dev_dependencies:
   flutter_test:
     sdk: flutter
   flutter_lints: ^5.0.0
+  drift_dev: ^2.31.0
+  build_runner: ^2.15.1
+  sqlite3: ^2.9.4
 
 flutter:
   uses-material-design: true