Refactor database layer: convert to DatabaseProvider class with initialization

bin/models/user.dart

@@ -0,0 +1,26 @@
+class User {
+  final int id;
+  final String login;
+  final String pwdHash;
+  
+  User({
+    required this.id,
+    required this.login,
+    required this.pwdHash,
+  });
+  
+  Map<String, dynamic> toMap() {
+    return {
+      'id': id,
+      'login': login,
+    };
+  }
+  
+  factory User.fromMap(Map<String, dynamic> map) {
+    return User(
+      id: map['id'],
+      login: map['login'],
+      pwdHash: map['pwd_hash'],
+    );
+  }
+}