fix: use contrast threshold for saturated background colors

[hervedombya]

Problem

Our getContrastText utility picks a text color (light or dark) to display on a given background. It was using a pure "best contrast wins" approach — it computed the WCAG contrast ratio for both candidate text colors against the background and returned whichever was higher.

This breaks on saturated mid-luminance colors. Some colors have a deceptively low WCAG relative luminance despite appearing visually bright, so the algorithm would pick dark text — which looks wrong and is harder to read in practice. This is a well-known limitation of WCAG luminance calculation with saturated reds and greens.

Solution

Replaced the "highest contrast wins" comparison with a threshold-based approach — the same strategy MUI (Material UI) uses:

1. Compute the contrast ratio of the lighter text candidate against the background.
2. If it meets a minimum contrast threshold (3.0 — WCAG AA for large text), use the lighter text.
3. Otherwise, fall back to the darker text.

This biases toward light text on saturated colors, which matches human visual expectation and is consistent with how most design systems handle this case.

[bert-e]

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[bert-e]

Waiting for approval

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[hervedombya]

/approve

[bert-e]

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[hervedombya]

/approve

[bert-e]

I have successfully merged the changeset of this pull request
into targetted development branches:

• ✔️ development/1.0

Please check the status of the associated issue CUI-3.

Goodbye hervedombya.

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