Drag and drop clients and channels in the server tree

Clients can be dragged into a channel and channels re-parented or
reordered, with an insertion line (or a row outline, for "into this
channel") showing where the drop lands.

Channel positions come from channel_order, which is the id of the
channel above rather than an index, so siblings are now ordered by
walking that chain and the neighbour links are repaired locally on
create, move and delete -- the server only announces the channel
that changed.

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

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

@@ -292,6 +292,21 @@ final class ServerTab implements ConnectionListener, ServerTreePanel.Actions {
         if (conn.isConnected()) conn.joinChannel(channelId, null);
     }
 
+    @Override
+    public void moveClientToChannel(ClientEntry client, ChannelNode target) {
+        if (!conn.isConnected()) return;
+        if (client.id == conn.getSelfClientId()) {
+            conn.joinChannel(target.id, null);
+        } else {
+            conn.moveClient(client.id, target.id, null);
+        }
+    }
+
+    @Override
+    public void moveChannel(ChannelNode channel, int newParentId, int orderPredecessorId) {
+        if (conn.isConnected()) conn.moveChannel(channel.id, newParentId, orderPredecessorId);
+    }
+
     @Override
     public void openPrivateChat(ClientEntry client) {
         chatPanel.openPrivateChat(client.id, client.nickname);