feat: création services singleton et renommage Box

Services créés:
✅ CurrentUserService singleton pour utilisateur connecté
✅ CurrentAmicaleService singleton pour amicale courante
✅ ApiService transformé en singleton

Box Hive:
✅ Renommage users -> user (plus logique)
✅ Migration automatique des données
✅ Services intégrés dans main.dart

État: Services créés, prêt pour refactorisation repositories

app/build/web/flutter_bootstrap.js

@@ -39,6 +39,6 @@ _flutter.buildConfig = {"engineRevision":"1425e5e9ec5eeb4f225c401d8db69b860e0fde
 
 _flutter.loader.load({
   serviceWorkerSettings: {
-    serviceWorkerVersion: "2020305023"
+    serviceWorkerVersion: "3087561979"
   }
 });

app/build/web/flutter_service_worker.js

@@ -3,13 +3,13 @@ const MANIFEST = 'flutter-app-manifest';
 const TEMP = 'flutter-temp-cache';
 const CACHE_NAME = 'flutter-app-cache';
 
-const RESOURCES = {"flutter_bootstrap.js": "aac8cb24376ac6215b72a88087969ea7",
+const RESOURCES = {"flutter_bootstrap.js": "1dbbc20d0a6216228635de38eb5222a5",
 "version.json": "727f6f584c125faac83c6d2a4c96fb3d",
 "index.html": "2aab03d10fea3b608e3eddc0fc0077e5",
 "/": "2aab03d10fea3b608e3eddc0fc0077e5",
 "favicon-64.png": "259540a3217e969237530444ca0eaed3",
 "favicon-16.png": "106142fb24eba190e475dbe6513cc9ff",
-"main.dart.js": "00dc118c0abed215458c35d4ba5a89b1",
+"main.dart.js": "7530a148b4d52ca92f4706f660f16907",
 "flutter.js": "83d881c1dbb6d6bcd6b42e274605b69c",
 "favicon.png": "21510778ead066ac826ad69302400773",
 "icons/Icon-192.png": "f36879dd176101fac324b68793e4683c",