Tabbed chats with BBCode rendering and clickable identities

Replace the chat target dropdown with a tab strip along the bottom of the
chat panel: a Server and a Channel tab plus one closable tab per private
chat, each with its own log and an unread marker.

Render logs and channel/client descriptions as HTML through a new BBCode
renderer in core, supporting b/i/u/s, color, size, url, center and lists,
with auto-linked bare URLs. Input is escaped and tag values validated, so
server-supplied text cannot inject markup or unsafe schemes.

Sender names and TeamSpeak's own client:// links open the same context menu
as the server tree (menus extracted into ClientMenu/ChannelMenu); channel://
links open the channel menu. Links are styled by CSS class: identities bold
in the author colour, other TS protocols plain, and links leaving the client
underlined so they cannot be mistaken for an identity.

Dragging a client or channel out of the tree yields the TS3 link BBCode.

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

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

@@ -0,0 +1,28 @@
+package com.ts3client.ui;
+
+import com.ts3client.net.ChannelNode;
+
+import javax.swing.JMenuItem;
+import javax.swing.JPopupMenu;
+
+/**
+ * The context menu for a channel, shared by the server tree and channel links
+ * in the chat log.
+ */
+final class ChannelMenu {
+
+    private ChannelMenu() {
+    }
+
+    static JPopupMenu build(ChannelNode channel, ServerTreePanel.Actions actions) {
+        JPopupMenu menu = new JPopupMenu();
+        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);
+        return menu;
+    }
+}