Show and control channel subscriptions

Track the subscription state from the notifychannelsubscribed events, draw
subscribed and unsubscribed channels apart with the pack's icons (and the
built-in glyph as a hollow cone), and offer subscribe/unsubscribe entries —
channel and family — in the channel context menu.

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

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

@@ -354,6 +354,11 @@ final class ServerTab implements ConnectionListener, ServerTreePanel.Actions {
         new ConnectionInfoDialog(host, conn, client.id, client.nickname).setVisible(true);
     }
 
+    @Override
+    public void setChannelSubscribed(ChannelNode channel, boolean family, boolean subscribed) {
+        if (conn.isConnected()) conn.setChannelsSubscribed(channel.familyIds(family), subscribed);
+    }
+
     @Override
     public void browseFiles(ChannelNode channel) {
         if (!conn.canTransferFiles()) return;