feat(frontend): connect borrow flow to api

eme-frontend/lib/services/api_client.dart

@@ -37,6 +37,31 @@ class ApiClient {
     throw ApiException(_messageErreur(reponse));
   }
 
+  static Future<dynamic> post(String chemin, Map<String, dynamic> body) async {
+    final uri = Uri.parse("$_baseUrl$chemin");
+
+    late final http.Response reponse;
+    try {
+      reponse = await http.post(
+        uri,
+        headers: const {
+          "content-type": "application/json",
+          "x-user-email": _utilisateurEmail,
+        },
+        body: jsonEncode(body),
+      );
+    } catch (_) {
+      throw const ApiException(
+        "Impossible de joindre le serveur. Vérifiez que le backend est démarré.",
+      );
+    }
+
+    if (reponse.statusCode >= 200 && reponse.statusCode < 300) {
+      return jsonDecode(reponse.body);
+    }
+    throw ApiException(_messageErreur(reponse));
+  }
+
   static String _messageErreur(http.Response reponse) {
     try {
       final corps = jsonDecode(reponse.body);

eme-frontend/lib/services/emprunt_service.dart

@@ -0,0 +1,26 @@
+import "../models/emprunt_response.dart";
+import "api_client.dart";
+
+/// Appels liés aux emprunts.
+class EmpruntService {
+  EmpruntService._();
+
+  static const int posteEmpruntId = 1;
+
+  static Future<EmpruntResponse> creerEmprunt({
+    required int materielId,
+    required String commentaireDepart,
+    required List<Map<String, dynamic>> elements,
+  }) async {
+    final reponse = await ApiClient.post("/emprunts", {
+      "materielId": materielId,
+      "posteEmpruntId": posteEmpruntId,
+      "modeIdentification": "MAIL_ENSUP",
+      if (commentaireDepart.trim().isNotEmpty) "commentaireDepart": commentaireDepart.trim(),
+      "elements": elements,
+    });
+
+    final data = (reponse as Map<String, dynamic>)["data"] as Map<String, dynamic>;
+    return EmpruntResponse.fromJson(data);
+  }
+}