feat(auth): prepare MSAL browser integration

eme-frontend/lib/services/auth_service.dart

@@ -1,3 +1,5 @@
+import "msal_bridge.dart";
+
 enum AuthenticationMode { demo, azure }
 
 class AuthService {
@@ -17,6 +19,7 @@ class AuthService {
   static const String demoResponsableEmail = "karim.benali@ensup.eu";
 
   static String? _demoEmail;
+  static bool _azureInitialized = false;
 
   static AuthenticationMode get mode {
     return switch (_configuredMode) {
@@ -40,9 +43,31 @@ class AuthService {
     _demoEmail = null;
   }
 
-  static Map<String, String> requestHeaders() {
+  static Future<void> signInWithMicrosoft() async {
+    await _initializeAzure();
+    await MsalBridge.signIn();
+  }
+
+  static Future<bool> hasMicrosoftAccount() async {
+    await _initializeAzure();
+    return MsalBridge.hasAccount();
+  }
+
+  static Future<void> signOut() async {
     if (mode == AuthenticationMode.azure) {
-      throw StateError("La session Microsoft n'est pas encore initialisee.");
+      await _initializeAzure();
+      await MsalBridge.signOut();
+      return;
+    }
+
+    clearSession();
+  }
+
+  static Future<Map<String, String>> requestHeaders() async {
+    if (mode == AuthenticationMode.azure) {
+      await _initializeAzure();
+      final token = await MsalBridge.getAccessToken();
+      return {"authorization": "Bearer $token"};
     }
 
     final email = _demoEmail;
@@ -53,6 +78,28 @@ class AuthService {
     return {"x-user-email": email};
   }
 
+  static Future<void> _initializeAzure() async {
+    if (mode != AuthenticationMode.azure) {
+      throw StateError("MSAL est disponible uniquement en mode Azure.");
+    }
+    if (_azureInitialized) {
+      return;
+    }
+    if (azureTenantId.isEmpty ||
+        azureSpaClientId.isEmpty ||
+        azureApiScope.isEmpty) {
+      throw StateError("La configuration Azure du frontend est incomplete.");
+    }
+
+    await MsalBridge.initialize(
+      tenantId: azureTenantId,
+      clientId: azureSpaClientId,
+      scope: azureApiScope,
+      redirectUri: Uri.base.origin,
+    );
+    _azureInitialized = true;
+  }
+
   static void _ensureDemoMode() {
     if (mode != AuthenticationMode.demo) {
       throw StateError(