feat: implement authentication flow with AppStartGate, AuthGuard, and calendar page initialization

em2rp/lib/views/calendar_page.dart

@@ -7,6 +7,7 @@ import 'package:em2rp/utils/performance_monitor.dart';
 import 'package:flutter/material.dart';
 import 'package:em2rp/views/widgets/nav/custom_app_bar.dart';
 import 'package:em2rp/views/widgets/nav/main_drawer.dart';
+import 'package:em2rp/views/widgets/common/startup_splash_screen.dart';
 import 'package:provider/provider.dart';
 import 'package:table_calendar/table_calendar.dart';
 import 'package:em2rp/models/event_model.dart';
@@ -864,11 +865,7 @@ class _CalendarPageState extends State<CalendarPage> {
     }
 
     if (eventProvider.isLoading) {
-      return const Scaffold(
-        body: Center(
-          child: CircularProgressIndicator(),
-        ),
-      );
+      return const StartupSplashScreen(message: 'Chargement des événements...');
     }
 
     return Scaffold(