feat(auth): Add OAuth providers and improve authentication flow

lib/core/models/server_config.dart

@@ -1,8 +1,24 @@
+import 'package:flutter/foundation.dart';
 import 'package:freezed_annotation/freezed_annotation.dart';
 
+import 'backend_config.dart';
+
 part 'server_config.freezed.dart';
 part 'server_config.g.dart';
 
+/// Container for passing server and backend config during authentication flow.
+@immutable
+class AuthFlowConfig {
+  const AuthFlowConfig({required this.serverConfig, this.backendConfig});
+
+  /// The server configuration (URL, headers, etc.).
+  final ServerConfig serverConfig;
+
+  /// The backend configuration (auth methods, features, etc.).
+  /// May be null if not yet fetched.
+  final BackendConfig? backendConfig;
+}
+
 @freezed
 sealed class ServerConfig with _$ServerConfig {
   const factory ServerConfig({