Stage 2: Library search, filters and Exercise detail with Flip Animation

- LibraryFilter: word search on names; muscle (primary), equipment and
  level choices that widen within a filter and narrow across filters
- ExerciseBrowser (reusable for the Plan picker) with search field,
  filter sheets showing live result counts, a clear button and an
  empty state
- Exercise detail: start/end Flip Animation at each Mode's pace (still
  with reduced motion), facts, main/secondary muscles, numbered steps

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

test/support/test_app.dart

@@ -1,3 +1,4 @@
+import 'package:flutter/widgets.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:provider/provider.dart';
 import 'package:shared_preferences/shared_preferences.dart';
@@ -7,25 +8,51 @@ import 'package:zenxercise/src/exercises/exercise_library.dart';
 import 'package:zenxercise/src/modes/mode.dart';
 import 'package:zenxercise/src/settings/settings_controller.dart';
 
-Exercise exercise(String id, Mode mode, {String? name, int imageCount = 0}) => Exercise(
+Exercise exercise(
+  String id,
+  Mode mode, {
+  String? name,
+  List<String> primary = const ['hamstrings'],
+  List<String> secondary = const [],
+  String? equipment = 'body only',
+  String level = 'beginner',
+  List<String> instructions = const ['Breathe.'],
+  int imageCount = 0,
+}) => Exercise(
   id: id,
   name: name ?? id.replaceAll('_', ' '),
   mode: mode,
   category: mode == Mode.stretching ? 'stretching' : 'strength',
-  level: 'beginner',
-  equipment: 'body only',
-  primaryMuscles: const ['hamstrings'],
-  secondaryMuscles: const [],
-  instructions: const ['Breathe.'],
+  level: level,
+  equipment: equipment,
+  primaryMuscles: primary,
+  secondaryMuscles: secondary,
+  instructions: instructions,
   imageCount: imageCount,
 );
 
+/// Two stretches and a squat. The 90/90 Hamstring's images exist in the real assets.
 final sampleLibrary = ExerciseLibrary([
-  exercise('Hamstring_Stretch', Mode.stretching),
-  exercise('Calf_Stretch', Mode.stretching),
-  exercise('Barbell_Squat', Mode.workout),
+  exercise(
+    '90_90_Hamstring',
+    Mode.stretching,
+    name: '90/90 Hamstring',
+    secondary: const ['calves'],
+    instructions: const ['Lie on your back.', 'Straighten one leg.'],
+    imageCount: 2,
+  ),
+  exercise('Calf_Stretch', Mode.stretching, primary: const ['calves'], equipment: 'other', level: 'intermediate'),
+  exercise('Barbell_Squat', Mode.workout, primary: const ['quadriceps'], equipment: 'barbell'),
 ]);
 
+/// A Pixel-sized screen instead of the default 800×600, so the floating
+/// navigation bar does not cover what a test taps.
+void usePhoneScreen(WidgetTester tester) {
+  tester.view.physicalSize = const Size(1080, 2400);
+  tester.view.devicePixelRatio = 2.75;
+  addTearDown(tester.view.reset);
+}
+
 /// The whole app over [library], with preferences starting from [prefs].
 Future<SettingsController> pumpApp(
   WidgetTester tester, {