Open text chat by double-clicking a client

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

ts3-client/swing/src/main/java/com/ts3client/ui/ServerTreePanel.java

@@ -155,6 +155,8 @@ public final class ServerTreePanel extends JScrollPane {
                 if (SwingUtilities.isLeftMouseButton(e) && e.getClickCount() == 2) {
                     if (obj instanceof ChannelNode && !Spacers.isSpacer(((ChannelNode) obj).name)) {
                         actions.joinChannel(((ChannelNode) obj).id);
+                    } else if (obj instanceof ClientEntry) {
+                        actions.openPrivateChat((ClientEntry) obj);
                     }
                 } else if (SwingUtilities.isMiddleMouseButton(e) && obj instanceof ClientEntry) {
                     actions.showConnectionInfo((ClientEntry) obj);