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/MainFrame.java

@@ -469,6 +469,12 @@ public final class MainFrame extends JFrame implements ConnectionListener, Serve
         new ConnectionInfoDialog(this, conn, client.id, client.nickname).setVisible(true);
     }
 
+    @Override
+    public void browseFiles(ChannelNode channel) {
+        if (!conn.canTransferFiles()) return;
+        new FileBrowserDialog(this, conn, channel).setVisible(true);
+    }
+
     @Override
     public boolean isClientLocallyMuted(int clientId) {
         return conn.getPlayback() != null && conn.getPlayback().isClientMuted(clientId);