Use Telegram Login Widget for web auth

lib/auth/telegram_session_web.dart

@@ -0,0 +1,21 @@
+import 'dart:js_interop';
+
+import 'package:web/web.dart' as web;
+
+const telegramLoginStorageKey = 'mapflow.telegramLoginData';
+
+@JS('window.Telegram.WebApp.initData')
+external JSString? get _telegramInitData;
+
+@JS('JSON.stringify')
+external JSString _jsonStringify(JSAny? value);
+
+String telegramInitData() => _telegramInitData?.toDart ?? '';
+
+String telegramLoginData() =>
+    web.window.localStorage.getItem(telegramLoginStorageKey) ?? '';
+
+void saveTelegramLoginData(JSAny? user) {
+  final encoded = _jsonStringify(user).toDart;
+  web.window.localStorage.setItem(telegramLoginStorageKey, encoded);
+}