Replace Telegram widget with bot login

lib/auth/telegram_login_button.dart

@@ -1,2 +1,26 @@
-export 'telegram_login_button_stub.dart'
-    if (dart.library.html) 'telegram_login_button_web.dart';
+import 'package:flutter/material.dart';
+
+class TelegramLoginButton extends StatelessWidget {
+  const TelegramLoginButton({
+    required this.onPressed,
+    required this.loading,
+    super.key,
+  });
+
+  final VoidCallback? onPressed;
+  final bool loading;
+
+  @override
+  Widget build(BuildContext context) {
+    return FilledButton.icon(
+      onPressed: loading ? null : onPressed,
+      icon: loading
+          ? const SizedBox.square(
+              dimension: 18,
+              child: CircularProgressIndicator(strokeWidth: 2),
+            )
+          : const Icon(Icons.telegram),
+      label: const Text('Войти через Telegram'),
+    );
+  }
+}