Register Channel tab's JFXPanel scenes with the ThemeManager

The Channel tab embeds two JavaFX scenes inside Swing via JFXPanel: the
noise squelch chart (NoiseSquelchView) and the symbol view.  Neither was
registered with the ThemeManager, so the dark stylesheet was never applied
to them.  In dark mode the chart legend, axis area and surrounding panel
rendered with JavaFX's default light Modena styling against the otherwise
dark UI.

JFXPanel-hosted scenes are invisible to javafx.stage.Window.getWindows(),
so the theme manager cannot find them automatically on toggle - they have
to be explicitly registered at creation time, the same as the JavaFX
Stages JavaFxWindowManager already registers.

src/main/java/io/github/dsheirer/gui/channel/ChannelSpectrumPanel.java

@@ -24,6 +24,7 @@ import io.github.dsheirer.dsp.filter.channelizer.PolyphaseChannelSource;
 import io.github.dsheirer.gui.power.SignalPowerView;
 import io.github.dsheirer.gui.squelch.NoiseSquelchView;
 import io.github.dsheirer.gui.symbol.SymbolView;
+import io.github.dsheirer.gui.theme.ThemeManager;
 import io.github.dsheirer.module.ProcessingChain;
 import io.github.dsheirer.module.decode.FeedbackDecoder;
 import io.github.dsheirer.module.decode.PrimaryDecoder;
@@ -200,6 +201,7 @@ public class ChannelSpectrumPanel extends JPanel implements Listener<ProcessingC
         //Spin noise squelch panel construction off onto the JavafX UI thread.
         Platform.runLater(() -> {
             Scene scene = new Scene(mNoiseSquelchView);
+            ThemeManager.getInstance().register(scene);
             mNoiseSquelchPanel.setScene(scene);
             Scene scene2 = new Scene(mSymbolView);
             URL resource = getClass().getResource("/sdrtrunk_style.css");
@@ -213,6 +215,7 @@ public class ChannelSpectrumPanel extends JPanel implements Listener<ProcessingC
                 LOGGER.warn("Can't find stylesheet resource for sdrtrunk");
             }
 
+            ThemeManager.getInstance().register(scene2);
             mSymbolPanel.setScene(scene2);
         });