Sunday530/dsh-sidebar-panel-collapse

Adds a chevron row under New Session that folds the sidebar panel list in place, returning the height to the workspace and session list. The hiding is driven by one attribute on the page root plus CSS: no host bundle is patched and no row is unregistered, and plugin entries injected outside the panel slot fold too because they are matched by their own data attributes. The state is remembered per browser and the control hides itself in the 56px rail.