Show client avatars and let the user set their own

Avatars are fetched from the server's file repository as /avatar_<id>,
verified against client_flag_avatar and cached on disk; animated GIFs play in
the info panel. Self > Set avatar... uploads or removes our own picture within
the server's size limit.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

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

@@ -369,6 +369,10 @@ final class ServerTab implements ServerTabConnectionEvents.Listener {
         if (conn.isConnected()) conn.setNickname(nickname);
     }
 
+    void showAvatarDialog() {
+        if (conn.isConnected()) new AvatarDialog(host, conn).setVisible(true);
+    }
+
     // ---- chat ----
 
     private void onSendChat(ChatPanel.Target target, int clientId, String text) {