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>

lib/src/app.dart

@@ -4,6 +4,7 @@ import 'package:provider/provider.dart';
 
 import 'design/app_theme.dart';
 import 'design/zen_style.dart';
+import 'exercises/views/exercise_detail_view.dart';
 import 'home/home_view.dart';
 import 'localization/app_localizations.dart';
 import 'settings/settings_controller.dart';
@@ -29,8 +30,13 @@ class ZenXerciseApp extends StatelessWidget {
       ],
       supportedLocales: AppLocalizations.supportedLocales,
       onGenerateTitle: (context) => AppLocalizations.of(context).appTitle,
-      onGenerateRoute: (settings) =>
-          MaterialPageRoute<Object?>(settings: settings, builder: (context) => const HomeView()),
+      onGenerateRoute: (settings) => MaterialPageRoute<Object?>(
+        settings: settings,
+        builder: (context) => switch (settings.name) {
+          ExerciseDetailView.routeName => ExerciseDetailView(exerciseId: settings.arguments! as String),
+          _ => const HomeView(),
+        },
+      ),
     );
   }
 }