fix: 通道图例不勾选时显示灰色方块+边框，勾选立即刷新

- 图例方块：勾选=彩色，不勾选=深灰+白边框，一目了然
- setChannelVisible 立即 update() 不再等定时器，消除点击延迟

plotwidget.cpp

@@ -101,6 +101,7 @@ void PlotWidget::setChannelVisible(int channel, bool visible)
     if (channel >= 0 && channel < 16) {
         m_channelVisible[channel] = visible;
         m_dirty = true;
+        update();  // 立即刷新，不等定时器
     }
 }
 
@@ -381,13 +382,17 @@ void PlotWidget::drawLegend(QPainter &painter)
     for (int ch = 0; ch < 16; ++ch) {
         int y = legendY + ch * itemHeight;
 
-        // 颜色方块
-        QColor color = m_channelVisible[ch] ? m_colors[ch] : QColor("#444444");
+        // 颜色方块：勾选=彩色，不勾选=灰白
+        QColor color = m_channelVisible[ch] ? m_colors[ch] : QColor("#555555");
         painter.fillRect(QRect(legendX, y + 2, 12, 12), color);
+        if (!m_channelVisible[ch]) {
+            painter.setPen(QPen(QColor("#888888"), 1));
+            painter.drawRect(QRect(legendX, y + 2, 12, 12));
+        }
 
         // 通道名
         QString label = QString("ch%1").arg(ch);
-        painter.setPen(m_channelVisible[ch] ? m_textColor : QColor("#666666"));
+        painter.setPen(m_channelVisible[ch] ? m_textColor : QColor("#555555"));
         painter.drawText(QRect(legendX + 16, y, 50, itemHeight),
                          Qt::AlignLeft | Qt::AlignVCenter, label);
     }