Stage 1: project setup, Exercise Library, both Mode designs and the Mode switch

- Flutter Android app on CookIT's stack (provider, shared_preferences, gen-l10n)
- tools/exercise-import turns free-exercise-db into 876 bundled Exercises
  with 1,746 WebP images (20 MB); stretching -> Stretching, rest -> Workout
- ZenStyle tokens per Mode: calm rounded sage/teal with breath ripples vs.
  dark ember-to-red with floor-tape slashes; the theme morphs at each
  Mode's own pace
- Remembered global Mode switch, Plans / Library / History tabs

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

test/exercises/exercise_library_test.dart

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+import 'package:flutter/services.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:zenxercise/src/exercises/exercise_library.dart';
+import 'package:zenxercise/src/modes/mode.dart';
+
+void main() {
+  TestWidgetsFlutterBinding.ensureInitialized();
+
+  const json = '''[
+    {"id":"Calf_Stretch","name":"Calf Stretch","mode":"stretching","category":"stretching","level":"beginner",
+     "equipment":null,"force":"static","mechanic":null,"primaryMuscles":["calves"],"secondaryMuscles":[],
+     "instructions":["Lean forward."],"imageCount":2},
+    {"id":"Barbell_Squat","name":"Barbell Squat","mode":"workout","category":"strength","level":"intermediate",
+     "equipment":"barbell","force":"push","mechanic":"compound","primaryMuscles":["quadriceps"],
+     "secondaryMuscles":["glutes","hamstrings"],"instructions":["Squat."],"imageCount":0}
+  ]''';
+
+  test('splits Exercises by Mode and finds them by id', () {
+    final library = ExerciseLibrary.fromJson(json);
+
+    expect(library.forMode(Mode.stretching).map((e) => e.id), ['Calf_Stretch']);
+    expect(library.forMode(Mode.workout).map((e) => e.id), ['Barbell_Squat']);
+    expect(library.byId('Barbell_Squat')!.secondaryMuscles, ['glutes', 'hamstrings']);
+    expect(library.byId('Missing'), isNull);
+  });
+
+  test('an Exercise points at its flattened WebP images', () {
+    final calf = ExerciseLibrary.fromJson(json).byId('Calf_Stretch')!;
+
+    expect(calf.images, ['assets/exercises/images/Calf_Stretch_0.webp', 'assets/exercises/images/Calf_Stretch_1.webp']);
+  });
+
+  test('the bundled library holds 123 stretches and 753 workout Exercises whose images exist', () async {
+    final library = await ExerciseLibrary.load(rootBundle);
+
+    expect(library.forMode(Mode.stretching), hasLength(123));
+    expect(library.forMode(Mode.workout), hasLength(753));
+    final anyImage = library.forMode(Mode.stretching).firstWhere((e) => e.imageCount == 2).images.last;
+    expect((await rootBundle.load(anyImage)).lengthInBytes, greaterThan(0));
+  });
+}