Fix initial theme applied to stories in storybook

[arome]

Before	After

[Copilot]

Pull Request Overview

This PR fixes the initial theme application in Storybook by replacing a custom dark mode implementation with the official storybook-dark-mode hook. The custom implementation relied on listening to channel events which didn't properly handle initial state, causing stories to render with incorrect themes on first load.

• Replaced custom useTheme hook with useDarkMode from storybook-dark-mode package
• Upgraded @itwin/itwinui-react to version ^3.19.4 to support the theme fix
• Removed unused React import and channel-based event listener implementation

Reviewed Changes

Copilot reviewed 2 out of 3 changed files in this pull request and generated no comments.

File	Description
packages/apps/storybook/package.json	Updated @itwin/itwinui-react dependency to ^3.19.4
packages/apps/storybook/.storybook/preview.js	Replaced custom theme hook with useDarkMode from storybook-dark-mode and removed unused imports

Files not reviewed (1)

• common/config/rush/pnpm-lock.yaml: Language not supported

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