Add undo support to the editor

[JustABiologist]

Summary: Adds Ctrl/Cmd+Z undo support backed by the existing NFA view serialization. Records checkpoints before structural editor changes including state creation/deletion, state movement, transition edits, accepting-state changes, start-state changes, and label edits. Clears transient selection/input state after an undo so restored automata render cleanly. Verification: node -c site/js/editor.js; node-based smoke test for undoing state creation, accepting toggle, state rename, and transition label edit. Closes #23. If accepted for the listed bounty, PayPal payout can be sent to stab.me.papi@gmail.com.

[JustABiologist]

Manual test matrix for the undo bounty:

• Create a state, undo, and confirm the state is removed.
• Toggle accepting state, undo, and confirm the accepting flag is restored.
• Rename a state, undo, and confirm the original label is restored.
• Edit a transition label, undo, and confirm the original label is restored.
• Syntax check passed for site/js/editor.js; I also ran a Node smoke test across these undo paths.