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/MainFrame.java

@@ -194,6 +194,11 @@ public final class MainFrame extends JFrame implements ServerTabPane.Listener {
                 changeNickname();
             }
 
+            @Override
+            public void onSetAvatar() {
+                if (selected != null && selected.isConnected()) selected.showAvatarDialog();
+            }
+
             @Override
             public void onShowIdentities() {
                 showIdentities();