fix: add mode-watcher to Sonner registry deps

[huntabyte]

The Sonner component needs us to specify a theme, otherwise it will default to the system preference. If the <ModeWatcher /> component is not being used, it will also default to "system". If you decide you do not want to use mode-watcher, you can simply remove it from your component and replace it with whatever you'd like.

This is aligned with how the original shadcn/ui's installation works, where next-themes is installed to keep the toast theme in sync with your app's theme.

Fixes: #595

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