refactor: 去掉多余的显示点数控件，来一个点画一个点，自动滚动

- 移除SpinBox和onDisplayPointsChanged
- 内部滚动窗口固定3000个点，数据无限追加
- 收到UDP包立即刷新绘图

mainwindow.cpp

@@ -79,29 +79,12 @@ void MainWindow::setupUi()
     );
     connect(m_clearBtn, &QPushButton::clicked, this, &MainWindow::onClear);
 
-    QLabel *displayLabel = new QLabel("显示点数:");
-    displayLabel->setStyleSheet("color: #ccc; font-size: 13px;");
-
-    m_displaySpinBox = new QSpinBox();
-    m_displaySpinBox->setRange(50, 10000);
-    m_displaySpinBox->setValue(500);
-    m_displaySpinBox->setSingleStep(100);
-    m_displaySpinBox->setStyleSheet(
-        "QSpinBox { background: #2a2a3e; color: #fff; border: 1px solid #555; "
-        "border-radius: 4px; padding: 4px; font-size: 13px; }"
-    );
-    connect(m_displaySpinBox, static_cast<void(QSpinBox::*)(int)>(&QSpinBox::valueChanged),
-            this, &MainWindow::onDisplayPointsChanged);
-
     controlLayout->addWidget(portLabel);
     controlLayout->addWidget(m_portEdit);
     controlLayout->addWidget(m_startStopBtn);
     controlLayout->addSpacing(20);
     controlLayout->addWidget(m_openFileBtn);
     controlLayout->addWidget(m_clearBtn);
-    controlLayout->addSpacing(20);
-    controlLayout->addWidget(displayLabel);
-    controlLayout->addWidget(m_displaySpinBox);
     controlLayout->addStretch();
 
     mainLayout->addLayout(controlLayout);
@@ -321,11 +304,6 @@ void MainWindow::processDataLine(const QString &line)
     m_packetLabel->setText(QString("收到: %1 包").arg(m_packetCount));
 }
 
-void MainWindow::onDisplayPointsChanged(int count)
-{
-    m_plotWidget->setDisplayPointCount(count);
-}
-
 void MainWindow::onToggleChannel(int channel, bool visible)
 {
     m_plotWidget->setChannelVisible(channel, visible);