Add the Create Channel dialog

The official client creates and edits channels with one dialog, so
ChannelEditDialog becomes ChannelDialog with two entry points. Creating
starts from caller-chosen defaults instead of a channelinfo and sends a
channelcreate carrying only what deviates from a plain new channel —
TeamSpeak checks a create permission per property that is present.

The server row now has a context menu with "Create Channel" and "Create
Spacer" (pre-filled with the lowest free [spacerN], since channel names
must be unique), and a channel's menu offers "Create Channel" and
"Create Sub-Channel", both starting out temporary.

Verified against the test server: a temporary channel is created, and a
permanent one a guest may not create reports the server's refusal.

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

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

@@ -1,6 +1,7 @@
 package com.ts3client.ui;
 
 import com.ts3client.net.ChannelNode;
+import com.ts3client.net.ChannelSettings;
 import com.ts3client.net.ClientEntry;
 import com.ts3client.net.Group;
 import com.ts3client.net.ServerModel;
@@ -47,6 +48,20 @@ public final class ServerTreePanel extends JScrollPane {
         /** Opens the channel editor for a channel. */
         void editChannel(ChannelNode channel);
 
+        /**
+         * Opens the channel creator.
+         *
+         * @param parent the channel the new one goes below, or {@code null} for a top-level one
+         * @param type   the channel type to start with
+         */
+        void createChannel(ChannelNode parent, ChannelSettings.Type type);
+
+        /** Opens the channel creator pre-filled as a spacer, which is always top-level. */
+        void createSpacer();
+
+        /** Whether the server is there to act on at all. */
+        boolean isConnected();
+
         /** Open the file repository browser for a channel. */
         void browseFiles(ChannelNode channel);
 
@@ -196,6 +211,9 @@ public final class ServerTreePanel extends JScrollPane {
             showClientMenu((ClientEntry) obj, e);
         } else if (obj instanceof ChannelNode) {
             showChannelMenu((ChannelNode) obj, e);
+        } else if (path.getPathCount() == 1 && actions.isConnected()) {
+            // The root row is the server itself, whose user object is just its name.
+            ServerMenu.build(actions).show(tree, e.getX(), e.getY());
         }
     }