Fix eslint-plugin-react-hooks v7 immutability violations

[Copilot]

The v7 upgrade introduces stricter immutability checks that flag variable reassignments in event handlers as violations.

Changes

• FormField.js: Replace variable reassignments with local copies in addConstraint and addProperty functions

The pattern changed from:

function addConstraint() {
  if (!validation) {
    validation = createElement(...);  // ❌ reassignment after render
  }
  // use validation
}

To:

function addConstraint() {
  let ensuredValidation = validation;
  if (!ensuredValidation) {
    ensuredValidation = createElement(...);  // ✅ local variable
  }
  // use ensuredValidation
}

Same pattern applied to properties variable in addProperty function.

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[Copilot]

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• googlechromelabs.github.io
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