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

em2rp/lib/utils/auth_guard_widget.dart

@@ -3,6 +3,7 @@ import 'package:em2rp/views/login_page.dart';
 import 'package:firebase_auth/firebase_auth.dart';
 import 'package:flutter/material.dart';
 import 'package:provider/provider.dart';
+import 'package:em2rp/views/widgets/common/startup_splash_screen.dart';
 
 class AuthGuard extends StatelessWidget {
   final Widget child;
@@ -33,12 +34,7 @@ class AuthGuard extends StatelessWidget {
         return child;
       }
 
-      return const Scaffold(
-        backgroundColor: Colors.white,
-        body: Center(
-          child: CircularProgressIndicator(),
-        ),
-      );
+      return const StartupSplashScreen(message: 'Chargement du profil...');
     }
 
     // Si l'utilisateur n'est pas connecté