Refactor LocalStorage

[masakudamatsu]

See how it should be done at https://epicreact.dev/modules/react-hooks/useeffect-persistent-state-solution

• And its subsequent videos as well for lazy state initialization and dependency array

Use JSON.stringify() to convert non-string state values into string values that are storable in localStorage.

• See https://epicreact.dev/modules/react-hooks/useeffect-persistent-state-extra-credit-solution-4