feat: refactor API paths for authentication, inventory, and recipes; add contract tests for repositories

flutter/lib/features/auth/data/auth_repository.dart

@@ -1,5 +1,6 @@
 import '../../../core/api/api_client.dart';
 import '../../../core/api/api_exception.dart';
+import '../../../core/api/api_paths.dart';
 import '../../../core/platform/token_storage.dart';
 
 class AuthRepository {
@@ -11,7 +12,7 @@ class AuthRepository {
   Future<String> login(String username, String password) async {
     try {
       final data = await _api.postJson(
-        '/auth/login',
+        AuthApiPaths.login,
         body: {'username': username, 'password': password},
       );