layout store (auto/custom + pending), marked custom on manual move (onDidMovePanel outside arrange), sash drag, stream/group rename. Tray chips: HTML5 drag accepted via onUnhandledDragOver/onDidDrop (position -> direction), click = new column. WebAudio two-note chime. Gateway page instructions updated. Verified headless (custom vs auto). Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>