feat(useragent): added connection info setup screen

useragent/lib/features/server_info_storage.dart

@@ -0,0 +1,67 @@
+import 'dart:convert';
+
+import 'package:flutter_secure_storage/flutter_secure_storage.dart';
+
+class StoredServerInfo {
+  const StoredServerInfo({
+    required this.address,
+    required this.port,
+    required this.caCertFingerprint,
+  });
+
+  final String address;
+  final int port;
+  final String caCertFingerprint;
+
+  Map<String, dynamic> toJson() => {
+    'address': address,
+    'port': port,
+    'caCertFingerprint': caCertFingerprint,
+  };
+
+  factory StoredServerInfo.fromJson(Map<String, dynamic> json) {
+    return StoredServerInfo(
+      address: json['address'] as String,
+      port: json['port'] as int,
+      caCertFingerprint: json['caCertFingerprint'] as String,
+    );
+  }
+}
+
+abstract class ServerInfoStorage {
+  Future<StoredServerInfo?> load();
+  Future<void> save(StoredServerInfo serverInfo);
+  Future<void> clear();
+}
+
+class SecureServerInfoStorage implements ServerInfoStorage {
+  static const _storageKey = 'server_info';
+
+  const SecureServerInfoStorage();
+
+  static const _storage = FlutterSecureStorage();
+
+  @override
+  Future<StoredServerInfo?> load() async {
+    final rawValue = await _storage.read(key: _storageKey);
+    if (rawValue == null) {
+      return null;
+    }
+
+    final decoded = jsonDecode(rawValue) as Map<String, dynamic>;
+    return StoredServerInfo.fromJson(decoded);
+  }
+
+  @override
+  Future<void> save(StoredServerInfo serverInfo) {
+    return _storage.write(
+      key: _storageKey,
+      value: jsonEncode(serverInfo.toJson()),
+    );
+  }
+
+  @override
+  Future<void> clear() {
+    return _storage.delete(key: _storageKey);
+  }
+}