feat(kline): 默认隐藏MA均线指标

- 将主图指标默认值从 0(MA) 改为 -1(无)
- 首次加载K线图时不再显示 MA5/MA10/MA20/MA60 均线
- 保留指标切换功能，用户可手动开启 MA/EMA/BOLL 指标
- 使K线图界面更简洁清爽

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/mining-app/lib/presentation/widgets/kline_chart/kline_chart_widget.dart

@@ -57,7 +57,8 @@ class _KlineChartWidgetState extends State<KlineChartWidget> {
   static const double _maxCandleWidth = 30.0;
 
   // 指标选择
-  int _selectedMainIndicator = 0; // 0: MA, 1: EMA, 2: BOLL
+  // -1: 无（不显示主图指标）, 0: MA, 1: EMA, 2: BOLL
+  int _selectedMainIndicator = -1;
   int _selectedSubIndicator = 0; // 0: MACD, 1: KDJ, 2: RSI
 
   // 十字线