The useStore subscriber called persistState on every state mutation,
triggering JSON.stringify + localStorage.setItem synchronously. It also
ran 8 localStorage.removeItem calls for legacy keys on every fire.

Wrap the subscriber with a 500ms debounce so rapid state changes batch
into a single write. Move legacy key cleanup behind a one-time flag so
it runs only once per page load. Add a beforeunload handler to flush
the final state.