Add pane swap control with persisted split layout preference

[jphavill]

Summary

• Add a small swap button (⇄) at the top of the split-pane divider to switch editor/preview sides.
• Persist pane swap preference across restarts using the existing frontend localStorage settings pattern.
• Keep divider drag-resize behavior correct after swapping by mapping ratio to the visible left/right panes.
• Update README to document the new swap control.

Why

I have dual vertical monitors and wanted to edit on the screen directly in front of me, while the preview was on the screen to my left. Other people may also prefer to edit on the right.

Implementation Notes

• Added panesSwapped state in app state.
• Loaded/saved inkwell-panes-swapped in settings.
• Added swap button to divider UI.
• Applied swapped pane ordering via CSS class on #app.
• Updated divider drag width assignment logic for swapped mode.

Validation

• npm run build passes successfully.

• Manually verified:

  • swap toggles pane sides,
  • preference persists after restart,
  • drag resize behaves correctly in both swapped and default layouts.