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/views/home_view_test.dart

@@ -16,7 +16,7 @@ void main() {
 
     expect(styleOf(tester).mode, Mode.stretching);
     expect(find.text('2 stretches'), findsOneWidget);
-    expect(find.text('Hamstring Stretch'), findsOneWidget);
+    expect(find.text('90/90 Hamstring'), findsOneWidget);
 
     await tester.tap(find.text('Workout'));
     await tester.pumpAndSettle();
@@ -26,7 +26,7 @@ void main() {
     expect(styleOf(tester).brightness, Brightness.dark);
     expect(find.text('1 exercise'), findsOneWidget);
     expect(find.text('Barbell Squat'), findsOneWidget);
-    expect(find.text('Hamstring Stretch'), findsNothing);
+    expect(find.text('90/90 Hamstring'), findsNothing);
   });
 
   testWidgets('opens in the last-used Mode', (tester) async {