feat(frontend): add back button in top bar (in-app navigation)

eme-frontend/lib/screens/home_screen.dart

@@ -41,10 +41,11 @@ class HomeScreen extends StatelessWidget {
                 clipBehavior: Clip.antiAlias,
                 child: Column(
                   children: [
-                    const EnsupTopBar(
+                    EnsupTopBar(
                       campusTag: "Paris · Ensitech",
                       userName: "Lucas Martin",
                       userInitials: "LM",
+                      onBack: () => Navigator.of(context).pop(),
                     ),
                     Expanded(
                       child: SingleChildScrollView(

eme-frontend/lib/widgets/ensup_top_bar.dart

@@ -2,8 +2,8 @@ import "package:flutter/material.dart";
 import "package:google_fonts/google_fonts.dart";
 import "../theme/ensup_colors.dart";
 
-/// Barre supérieure ENSUP. L'utilisateur est optionnel (absent sur l'écran
-/// d'identification, présent une fois l'étudiant connecté).
+/// Barre supérieure ENSUP. L'utilisateur et le bouton retour sont optionnels
+/// (absents sur l'écran d'identification).
 class EnsupTopBar extends StatelessWidget {
   const EnsupTopBar({
     super.key,
@@ -11,27 +11,34 @@ class EnsupTopBar extends StatelessWidget {
     required this.campusTag,
     this.userName,
     this.userInitials,
+    this.onBack,
   });
 
   final String brandText;
   final String campusTag;
   final String? userName;
   final String? userInitials;
+  final VoidCallback? onBack;
 
   @override
   Widget build(BuildContext context) {
     final name = userName;
     final initials = userInitials;
+    final onBack = this.onBack;
 
     return Container(
       height: 56,
-      padding: const EdgeInsets.symmetric(horizontal: 28),
+      padding: const EdgeInsets.symmetric(horizontal: 20),
       color: EnsupColors.blue3,
       child: Row(
         mainAxisAlignment: MainAxisAlignment.spaceBetween,
         children: [
           Row(
             children: [
+              if (onBack != null) ...[
+                _BackButton(onTap: onBack),
+                const SizedBox(width: 8),
+              ],
               const Icon(Icons.badge_outlined, color: Colors.white, size: 18),
               const SizedBox(width: 10),
               Text(
@@ -84,6 +91,38 @@ class EnsupTopBar extends StatelessWidget {
   }
 }
 
+class _BackButton extends StatelessWidget {
+  const _BackButton({required this.onTap});
+
+  final VoidCallback onTap;
+
+  @override
+  Widget build(BuildContext context) {
+    return InkWell(
+      onTap: onTap,
+      borderRadius: BorderRadius.circular(8),
+      child: Padding(
+        padding: const EdgeInsets.symmetric(horizontal: 10, vertical: 6),
+        child: Row(
+          mainAxisSize: MainAxisSize.min,
+          children: [
+            Icon(Icons.arrow_back, size: 16, color: Colors.white.withValues(alpha: 0.85)),
+            const SizedBox(width: 6),
+            Text(
+              "Retour",
+              style: GoogleFonts.titilliumWeb(
+                color: Colors.white.withValues(alpha: 0.85),
+                fontSize: 13,
+                fontWeight: FontWeight.w600,
+              ),
+            ),
+          ],
+        ),
+      ),
+    );
+  }
+}
+
 class _Avatar extends StatelessWidget {
   const _Avatar({required this.initials});