fix(sb10): wire dark/light mode switching in .storybook (v3/rac) preview
The @vueless/storybook-dark-mode toolbar toggle was a no-op in the v3/rac
Storybook because nothing in .storybook reacted to its DARK_MODE event.
- preview.js: subscribe to DARK_MODE_EVENT_NAME on the addons channel and
reflect the state on document.documentElement.dataset.colorScheme.
Initial value reads from the addon's `sb-addon-themes-3` localStorage key
so the first paint is correct (matches the .storybook-s2 pattern).
- custom-addons/provider/index.js: consume useDarkMode() and feed isDark
through to the React Spectrum Provider's `colorScheme` prop whenever the
ProviderSwitcher dropdown is set to "Auto". An explicit dropdown choice
still wins.