Implement channel file transfers

Add file up/downloads and directory operations for channel file
repositories, reachable from a channel's right-click "Browse files".

Core (com.ts3client.net.filetransfer):
- FileTransferManager drives the TS3 file protocol on the ts3j socket.
  ftinitupload/ftinitdownload results arrive out-of-band as
  notifystartupload/notifystartdownload (success) or
  notifystatusfiletransfer (failure) events, not as command responses,
  so negotiation correlates them by clientftfid via a CompletableFuture.
  The byte phase opens a raw TCP socket, sends the ftkey, then streams.
- FileTransfer: observable per-transfer handle (state, progress, cancel).
- RemoteFile: UI-facing listing DTO, keeping core free of ts3j API types.

TeamspeakConnection exposes list/mkdir/delete/rename and up/download and
manages the manager's lifecycle. Swing FileBrowserDialog provides the
browser UI with a live per-transfer progress strip.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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

@@ -37,6 +37,9 @@ public final class ServerTreePanel extends JScrollPane {
 
         void showConnectionInfo(ClientEntry client);
 
+        /** Open the file repository browser for a channel. */
+        void browseFiles(ChannelNode channel);
+
         boolean isClientLocallyMuted(int clientId);
 
         /** A channel or client node was selected (or {@code null} when cleared). */
@@ -152,6 +155,10 @@ public final class ServerTreePanel extends JScrollPane {
         JMenuItem join = new JMenuItem("Join channel");
         join.addActionListener(a -> actions.joinChannel(channel.id));
         menu.add(join);
+        menu.addSeparator();
+        JMenuItem files = new JMenuItem("Browse files");
+        files.addActionListener(a -> actions.browseFiles(channel));
+        menu.add(files);
         menu.show(tree, e.getX(), e.getY());
     }