feat(ui): écran Bibliothèque d'exercices

Ajoute l'écran de bibliothèque d'exercices (presentation/exercise_library_screen.dart)
et ajuste la couche application/domaine (bootstrap, ports, use cases,
entités, repositories Drift) pour l'alimenter. flutter analyze propre,
9/9 tests verts, build APK debug validé.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

lib/presentation/game_time_app.dart

@@ -1,6 +1,7 @@
 import 'package:flutter/material.dart';
 
 import '../application/app_bootstrap.dart';
+import 'exercise_library_screen.dart';
 
 final class GameTimeApp extends StatelessWidget {
   const GameTimeApp({required this.bootstrap, super.key});
@@ -15,8 +16,9 @@ final class GameTimeApp extends StatelessWidget {
         colorScheme: ColorScheme.fromSeed(seedColor: const Color(0xFF0E7C66)),
         useMaterial3: true,
       ),
-      home: const Scaffold(
-        body: SizedBox.shrink(),
+      home: ExerciseLibraryScreen(
+        exerciseUseCases: bootstrap.exerciseUseCases,
+        mediaUseCases: bootstrap.mediaUseCases,
       ),
     );
   }