Most React apps do not need Redux. What is the right default flow for picking where state should live? Sketch the lift-up step in code.

Build a ThemeContext with a provider and a useTheme hook. The hook should return { theme, toggle } and throw if called outside the provider.

Migrate this useState cart to useReducer. Why does the reducer version scale better as the cart grows more actions?

When does a lightweight store like Zustand or Jotai earn its keep over plain Context + useReducer? Be concrete about the failure mode it fixes.