Add dark mode to a Vite app

A small feature with acceptance criteria shows how to get predictable results from Build mode.

1. The prompt

Add dark mode:
- a toggle button in the header with an accessible label
- Tailwind "class" strategy; add the `dark` class on <html>
- respect prefers-color-scheme on first load, then remember the choice
  in localStorage
- no new dependencies
Done when `npm run build` passes and both themes render without unstyled flashes.

2. What to expect

The plan should mention tailwind.config (or the Tailwind v4 CSS variant the template uses), the header component and a tiny script that applies the stored theme before React renders. The validating stage runs the build.

3. Check it in the preview

Run the preview, toggle, reload. If the page flashes light before going dark, the theme script runs too late; ask in Fix mode with that sentence as the error description.

4. Adjust by hand

Open the component and change the icon or colours. Save; the preview updates.

5. Deploy

Deploy to Codexa hosting and test the live URL on a phone. Rollback is available if a later change regresses it.