feat(frontend): add student identification screen as entry point

eme-frontend/lib/widgets/ensup_top_bar.dart

@@ -2,21 +2,27 @@ import "package:flutter/material.dart";
 import "package:google_fonts/google_fonts.dart";
 import "../theme/ensup_colors.dart";
 
-/// Barre supérieure ENSUP : marque EME, campus et utilisateur connecté.
+/// Barre supérieure ENSUP. L'utilisateur est optionnel (absent sur l'écran
+/// d'identification, présent une fois l'étudiant connecté).
 class EnsupTopBar extends StatelessWidget {
   const EnsupTopBar({
     super.key,
+    this.brandText = "EME",
     required this.campusTag,
-    required this.userName,
-    required this.userInitials,
+    this.userName,
+    this.userInitials,
   });
 
+  final String brandText;
   final String campusTag;
-  final String userName;
-  final String userInitials;
+  final String? userName;
+  final String? userInitials;
 
   @override
   Widget build(BuildContext context) {
+    final name = userName;
+    final initials = userInitials;
+
     return Container(
       height: 56,
       padding: const EdgeInsets.symmetric(horizontal: 28),
@@ -26,10 +32,10 @@ class EnsupTopBar extends StatelessWidget {
         children: [
           Row(
             children: [
-              const Icon(Icons.devices_other, color: Colors.white, size: 18),
+              const Icon(Icons.badge_outlined, color: Colors.white, size: 18),
               const SizedBox(width: 10),
               Text(
-                "EME",
+                brandText,
                 style: GoogleFonts.darkerGrotesque(
                   color: Colors.white,
                   fontWeight: FontWeight.w900,
@@ -57,17 +63,19 @@ class EnsupTopBar extends StatelessWidget {
                   ),
                 ),
               ),
-              const SizedBox(width: 12),
-              _Avatar(initials: userInitials),
-              const SizedBox(width: 8),
-              Text(
-                userName,
-                style: GoogleFonts.titilliumWeb(
-                  color: Colors.white,
-                  fontSize: 13,
-                  fontWeight: FontWeight.w600,
+              if (name != null && initials != null) ...[
+                const SizedBox(width: 12),
+                _Avatar(initials: initials),
+                const SizedBox(width: 8),
+                Text(
+                  name,
+                  style: GoogleFonts.titilliumWeb(
+                    color: Colors.white,
+                    fontSize: 13,
+                    fontWeight: FontWeight.w600,
+                  ),
                 ),
-              ),
+              ],
             ],
           ),
         ],