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/GroupIcons.java

@@ -0,0 +1,101 @@
+package com.ts3client.ui;
+
+import com.ts3client.net.Group;
+import com.ts3client.net.IconRepository;
+
+import javax.swing.Icon;
+import javax.swing.ImageIcon;
+import java.awt.Component;
+import java.awt.Graphics;
+import java.awt.Image;
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Map;
+import java.util.concurrent.ConcurrentHashMap;
+
+/**
+ * Turns the {@link IconRepository}'s icon bytes into Swing icons for one server's
+ * groups, caching the decoded images. Icons that still have to be downloaded simply
+ * do not render yet; the repository repaints the view once they arrive.
+ */
+public final class GroupIcons {
+
+    /** Tree rows are 16×16 like TeamSpeak's own icon set. */
+    private static final int SIZE = 16;
+
+    private final IconRepository repository;
+    private final Map<Long, ImageIcon> decoded = new ConcurrentHashMap<>();
+
+    public GroupIcons(IconRepository repository) {
+        this.repository = repository;
+    }
+
+    /** @return the group's icon, or {@code null} if it has none or it is not loaded yet */
+    public ImageIcon iconOf(Group group) {
+        if (group == null || group.iconId == 0) return null;
+        return icon(group.iconId);
+    }
+
+    public ImageIcon icon(long iconId) {
+        ImageIcon cached = decoded.get(iconId);
+        if (cached != null) return cached;
+
+        byte[] data = repository.get(iconId);
+        if (data == null) return null;
+
+        ImageIcon icon = new ImageIcon(data);
+        if (icon.getIconWidth() <= 0) return null;
+        if (icon.getIconWidth() != SIZE || icon.getIconHeight() != SIZE) {
+            icon = new ImageIcon(icon.getImage().getScaledInstance(SIZE, SIZE, Image.SCALE_SMOOTH));
+        }
+        decoded.put(iconId, icon);
+        return icon;
+    }
+
+    /** Collects the icons of a client's server groups plus its channel group, in display order. */
+    public List<Icon> iconsOf(List<Group> serverGroups, Group channelGroup) {
+        List<Icon> icons = new ArrayList<>();
+        for (Group g : serverGroups) {
+            ImageIcon icon = iconOf(g);
+            if (icon != null) icons.add(icon);
+        }
+        ImageIcon channelIcon = iconOf(channelGroup);
+        if (channelIcon != null) icons.add(channelIcon);
+        return icons;
+    }
+
+    /** Lays several icons out in a row, so a single tree cell can show a badge strip. */
+    public static final class Row implements Icon {
+
+        private static final int GAP = 2;
+
+        private final List<Icon> icons;
+
+        public Row(List<Icon> icons) {
+            this.icons = icons;
+        }
+
+        @Override
+        public void paintIcon(Component c, Graphics g, int x, int y) {
+            int offset = x;
+            for (Icon icon : icons) {
+                icon.paintIcon(c, g, offset, y + (getIconHeight() - icon.getIconHeight()) / 2);
+                offset += icon.getIconWidth() + GAP;
+            }
+        }
+
+        @Override
+        public int getIconWidth() {
+            int width = 0;
+            for (Icon icon : icons) width += icon.getIconWidth() + GAP;
+            return Math.max(0, width - GAP);
+        }
+
+        @Override
+        public int getIconHeight() {
+            int height = 0;
+            for (Icon icon : icons) height = Math.max(height, icon.getIconHeight());
+            return height;
+        }
+    }
+}