Build a resizable split pane

• Two panes separated by a draggable handle, laid out horizontally (and vertically through a direction prop)
• Dragging uses pointer events with setPointerCapture, so it keeps working when the pointer leaves the handle or crosses an iframe
• Minimum and maximum sizes per pane are enforced, with the size clamped during dragging
• The size can be set in pixels or as a percentage, and the layout adapts correctly when the window resizes
• The size persists in localStorage and is restored on load (reads wrapped in try/catch)
• Keyboard: the handle is focusable with role="separator" and aria-orientation, arrow keys resize by a step, Home and End jump to the limits, and aria-valuenow is updated

Only after every requirement works.

• Double-click the handle to reset to the default size
• Collapse a pane with a button or by dragging past a threshold
• Support three or more panes