feat: replace mouse/touch events with pointer events

[tomkp]

Summary

• Replaces separate mouse and touch event handlers with unified Pointer Events API
• Implements pointer capture for better drag UX (prevents cursor blinking during resize)
• Simplifies the useResizer hook by consolidating event handling

Changes

• useResizer.ts: Replaced handleMouseDown, handleTouchStart, handleTouchEnd with single handlePointerDown. Updated global listeners to use pointermove, pointerup, pointercancel
• Divider.tsx: Changed from onMouseDown/onTouchStart/onTouchEnd to single onPointerDown prop
• SplitPane.tsx: Updated to use new handlePointerDown from useResizer hook
• types/index.ts: Updated DividerProps to use onPointerDown, changed ResizeEvent.source from 'mouse' | 'touch' to 'pointer'
• test/setup.ts: Added PointerEvent polyfill for jsdom testing

Breaking Changes

Custom divider components will need to update their props:

• Replace onMouseDown, onTouchStart, onTouchEnd with onPointerDown

Benefits

1. Better UX: Pointer capture prevents cursor blinking during drag
2. Simpler code: Single event handler instead of separate mouse/touch logic
3. Unified input: Handles mouse, touch, and pen input consistently
4. Modern standard: Pointer Events are the recommended approach for pointing device input

Browser Support

Pointer Events are supported in all modern browsers (IE11+, Chrome 55+, Firefox 59+, Safari 13+).

Test Plan

• All existing tests updated and passing
• New tests for pointer capture behavior
• Tests for touch and pen input via pointer events
• TypeScript type checking passes
• Linting and formatting pass

Closes #878