fix: maintain controlled pane sizes on container resize

[tomkp]

Summary

Fixes #868

When using controlled mode with pixel sizes, panes were incorrectly being scaled proportionally when the container/window resized. This fix checks for controlled size props in handleContainerSizeChange and uses calculateInitialSizes instead of distributeSizes to maintain the fixed sizes from props.

Changes

• Add hasControlledSizes check in handleContainerSizeChange
• Use calculateInitialSizes for controlled panes on resize (maintains fixed pixel sizes)
• Keep proportional distribution for uncontrolled panes (existing behavior)
• Add tests for container resize behavior
• Extend test setup to support triggering resize events

Root Cause

In SplitPane.tsx, handleContainerSizeChange unconditionally called distributeSizes() when the container resized, ignoring controlled size props. While there was a sync effect that attempted to restore controlled sizes, it:

• Caused unnecessary re-renders
• Could result in a visual "flash" of incorrect sizes
• Created potential race conditions in fast resize scenarios

Test Plan

• Run npm test - all tests pass
• Run npm run lint - no errors
• Run npm run typecheck - no errors
• Run npm run build - builds successfully
• Manual testing with the reproduction from Window resize issue with Controlled component #868