fix: Improve ButterBar text contrast on banners

[AAshGray]

Thanks for submitting a PR! Please check the boxes below:

• [ x ] I have read the Contributing Guide.
• [ x ] I have added information to docs/ if required so people know about the feature.
• [ x ] I have filled in the "Changes" section below.
• [ x ] I have filled in the "How did you test this code" section below.

Contributes to #7604

Changes

• Added Utils.getContrastColour(...) to frontend/common/utils/utils.tsx
• Updated frontend/web/components/ButterBar.tsx to use Utils.getContrastColour(...) for environment banner text colour instead of hardcoding white

How did you test this code?

• Ran the frontend locally
• Used an environment with a custom banner_colour and banner text
• Verified the banner text switched to black or white based on contrast

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Project	Deployment	Actions	Updated (UTC)
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[talissoncosta]

Thanks for the contribution! 🙏 Solid fix, reusable helper, WCAG luminance, the right 0.179 threshold, nicely scoped. The two notes above are minor and non-blocking. Looking forward to merging once they're sorted. 🎉

[AAshGray]

Thanks for the clear explanations, let me know if there's anything else! 😄

[talissoncosta]

Looks great! thanks for your contribution. 😃