feat: 错误自动上报机制

- 后端：error_reports 表 + POST /api/v1/public/errors + GET /api/v1/admin/errors
- Flutter：ErrorReporter 单例，60s 去重，fire-and-forget，不影响用户
- main.dart：FlutterError 和 Zone 错误自动上报
- 打印异常：catch 块加 reportError，Windows 打印问题可自动采集堆栈

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

client/lib/main.dart

@@ -4,6 +4,7 @@ import 'package:flutter_localizations/flutter_localizations.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 import 'package:flutter_web_plugins/url_strategy.dart';
 import 'core/auth/auth_state.dart';
+import 'core/errors/error_reporter.dart';
 import 'core/router/app_router.dart';
 import 'core/theme/app_theme.dart';
 import 'providers/connectivity_provider.dart';
@@ -15,6 +16,11 @@ void main() {
     debugPrint('═══ FlutterError ════════════════════════════');
     debugPrint(details.exceptionAsString());
     debugPrint(details.stack.toString());
+    ErrorReporter.instance.report(
+      errorType: ErrorType.flutterError,
+      errorMsg: details.exceptionAsString(),
+      stackTrace: details.stack,
+    );
   };
 
   runZonedGuarded(
@@ -23,6 +29,11 @@ void main() {
       debugPrint('═══ Zone Error ═══════════════════════════════');
       debugPrint(error.toString());
       debugPrint(stack.toString());
+      ErrorReporter.instance.report(
+        errorType: ErrorType.zoneError,
+        errorMsg: error.toString(),
+        stackTrace: stack,
+      );
     },
   );
 }