feat(account): 增加账号中心页面逻辑

lib/pages/account/account_export_logic.dart

@@ -0,0 +1,54 @@
+// Author: fruit.com.cn
+
+import 'package:flutter/foundation.dart';
+
+import '../../core/data/models/account_export.dart';
+import '../../core/data/remote/api_client.dart';
+import '../../core/data/repositories/account_repository.dart';
+
+/// 账号数据导出状态与复制动作。
+class AccountExportLogic extends ChangeNotifier {
+  AccountExportLogic(this._repository, {required this.copyText});
+
+  final AccountRepository _repository;
+  final Future<void> Function(String text) copyText;
+  AccountExport? data;
+  bool loading = false;
+  String? error;
+  bool _disposed = false;
+
+  /// 加载完整导出数据。
+  Future<void> load() async {
+    loading = true;
+    error = null;
+    _notify();
+    try {
+      data = await _repository.export();
+    } catch (failure) {
+      error = failure is ApiException
+          ? failure.message
+          : 'accountExportLoadFailed';
+    } finally {
+      loading = false;
+      _notify();
+    }
+  }
+
+  /// 把后端下发的完整 JSON 写入剪贴板。
+  Future<bool> copy() async {
+    final current = data;
+    if (current == null) return false;
+    await copyText(current.rawJson);
+    return true;
+  }
+
+  void _notify() {
+    if (!_disposed) notifyListeners();
+  }
+
+  @override
+  void dispose() {
+    _disposed = true;
+    super.dispose();
+  }
+}