Remove Sass legacy JS API build warnings

Problem

Running npm run build emits repeated Dart Sass deprecation warnings:

DEPRECATION WARNING [legacy-js-api]: The legacy JS API is deprecated and will be removed in Dart Sass 2.0.0.

The warning appears once for each SCSS compilation. The current build stack uses [email protected] with [email protected]; that loader calls Dart Sass through the legacy render() API.

Why it matters

Dart Sass 2.0.0 will remove the legacy JS API, so this is future breakage rather than just noisy output.

Acceptance criteria

• npm run build completes without any legacy-js-api Sass warnings.
• The SCSS/CSS output remains equivalent for the existing app.
• The fix keeps the CRA 3 build wrapper working on the current Node baseline.