Add pure-black POC to apps/storybook-react

[georgewrmarshall]

Description

Use apps/storybook-react as an extension-style proof of integration for the web provider and pure-black theme behavior.

Acceptance Criteria

• Storybook React is wrapped with PureBlackProvider
• A toggle/control exists for isPureBlack
• Standard dark and pure-black modes can be visually compared
• Surface components show correct hierarchy in pure-black mode

Constraint

• isPureBlack=false uses the existing grey dark theme
• isPureBlack=true uses pureBlackDarkTheme

References

• Jira epic: TMCU-622