fix: make ResizableHandle work with vertical orientation
The ResizableHandle component relied on data-[panel-group-direction=vertical]
selectors which don't work because react-resizable-panels doesn't propagate
the direction attribute to child components.
Add explicit orientation prop to ResizableHandle and pass it from AppShell
based on isNarrowScreen state. The vertical handle now renders with proper
height (h-2) and visible background.