Two-column bookmark manager with connect-on-startup

Replace the modal add/edit form with a master/detail dialog: bookmark
list on the left, the selected bookmark's settings on the right. Edits
are flushed back into the bookmark when the selection changes or the
dialog closes.

Bookmarks gain a "connect on startup" flag (only one at a time, since
the client holds a single connection); MainFrame connects to it once
the window is up.

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

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

@@ -141,6 +141,12 @@ public final class MainFrame extends JFrame implements ConnectionListener, Serve
         statusTimer.start();
         setSize(880, 560);
         setLocationRelativeTo(null);
+
+        Bookmark startup = bookmarks.startupBookmark();
+        if (startup != null) {
+            // Deferred so the window is on screen before we start connecting.
+            SwingUtilities.invokeLater(() -> connectToBookmark(startup));
+        }
     }
 
     // ---- UI construction ----