Environment banner text hardcoded to white, unreadable on light banner colours

## Describe the bug

The environment banner (the "butter bar" shown when `banner_text` is configured on an environment) renders its text in hardcoded white. When the chosen `banner_colour` is light, the text becomes effectively invisible against the background.

## To reproduce

1. Open an environment's settings
2. Enable the banner and set `banner_text` (e.g. `Production [Be Careful!]`)
3. Navigate to any page in that environment (Features, Segments, etc.)

The banner renders white-on-light and is unreadable.

## Expected behaviour

The banner text should remain legible regardless of the chosen `banner_colour`. Text color should be derived from the background.

## Affected code

`frontend/web/components/ButterBar.tsx` lines 73-85 hardcode `color: 'white'`:

## Screenshots

Will attach after issue creation (a banner with a pale cream `banner_colour` showing white text that is barely visible)

Start in frontend/web/components/ButterBar.tsx, especially lines 73-85, and reproduce the banner with a light banner_colour from environment settings. Replace the hardcoded white text behavior with a background-derived readable color, then verify the banner remains legible on light and dark colors across the listed environment pages.

Written by the indexing model from the issue text.