feat: add tabbed container layout mode with tab bar UI

Adds a third layout mode (TABBED) alongside HORIZONTAL and VERTICAL accordion modes.
In tabbed mode, only the active tab window is visible and a slim tab bar at the top
of the container shows all windows with click-to-switch. Keybinding: Ctrl+/ to toggle.

extension.ts

@@ -39,6 +39,7 @@ export default class aerospike extends Extension {
             'print-tree':         () => { this.windowManager.printTreeStructure(); },
             'toggle-orientation': () => { this.windowManager.toggleActiveContainerOrientation(); },
             'reset-ratios':       () => { this.windowManager.resetActiveContainerRatios(); },
+            'toggle-tabbed':      () => { this.windowManager.toggleActiveContainerTabbed(); },
         };
     }