Add undo/redo with input change batching

[Copilot]

Implements Cmd/Ctrl+Z undo and Shift+Cmd/Ctrl+Z redo. Input changes are batched while focused—multiple arrow key presses become a single undo stop on blur or Enter.

Implementation

History management (use-history.ts)

• Generic hook managing state stack with JSON-based change detection
• Tracks history index for bidirectional navigation

Focus detection (use-input-focus.ts)

• Disables global shortcuts when INPUT/TEXTAREA/contentEditable has focus
• Prevents conflict with native browser undo/redo

State integration (use-matrices.ts)

• Maintains separate display state for immediate feedback during skipHistory updates
• Syncs with history state on undo/redo via useEffect
• Commits pending input changes to history on blur

Keyboard handling (app.tsx)

useEffect(() => {
  const handleKeyDown = (e: KeyboardEvent) => {
    if (hasInputFocus) return
    
    const isModifierPressed = e.metaKey || e.ctrlKey
    if (isModifierPressed && e.key === 'z') {
      e.preventDefault()
      e.shiftKey ? values.redo() : values.undo()
    }
  }
  // ...
}, [hasInputFocus, values])

All operations are undoable: matrix updates, add/delete/move/invert/clone/toggle visibility.

Demo

Initial state with one matrix:

After adding second matrix:

After Cmd+Z (undo) removes second matrix:

Original prompt

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This section details on the original issue you should resolve

<issue_title>Undo/redo functionality</issue_title>
<issue_description>When an input doesn't have focus (those have their own notion of undo/redo), cmd z/shift cmd z (and ctrl variants on windows) should undo the last change made. I think all input changes should be batched while an input has focus. For example, if you are using arrowup/down to change the value of an input, each onChange event should not could as a single undo stop. Rather, the final value before the input is blurred should be considered the value. All operations should be undoable, including moving matrices around, inverting, etc. Probably easiest to store the entire state of all the matrices on every undo stop, guaranteeing we will always be able to go back to where we were previously.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Undo/redo functionality #9

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