Show server and channel group icons in the tree

Download the icons a server advertises for its groups over the file
transfer channel, cache them under the config directory, and draw them as
a badge strip on each client's row, right-aligned against the tree's
visible edge like the TeamSpeak client does. Groups without a custom icon
fall back to the bundled default set.

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

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

@@ -314,6 +314,7 @@ public final class MainFrame extends JFrame implements ServerTabPane.Listener {
         if (tabs.size() == 1) return; // always keep one view around
 
         tabs.remove(tab);
+        tab.dispose();
         tabPane.removeTab(tab);
         if (micTab == tab) micTab = null;
         if (selected == tab) {