Toggle theme provider

Context & Patterns — Beginner React challenge.

Wire up a small theme system with a context provider, a `useTheme` hook, and a toggle. Great warmup for the Context & Patterns track.

Requirements

• ThemeProvider stores the current theme in state (default "light").
• useTheme returns { theme, toggle } and throws when used outside the provider.
• toggle() flips between "light" and "dark".

Signature

type Theme = "light" | "dark";
function useTheme(): { theme: Theme; toggle: () => void };

Example

<ThemeProvider>
  <App />
</ThemeProvider>

Constraints

• No external state library.
• One provider per subtree.