Modernise SCSS: restore Dart Sass @use/@forward after GH Actions deploy migration

[mmcky]

Background

Blocked on #199 (switching production builds to GitHub Actions so Jekyll 4.4 is used in production).

The legacy GitHub Pages sandbox pins production to Jekyll 3.10 + LibSass. LibSass does not support Dart Sass @use/@forward directives, so when they were introduced in the site refresh (#194) they had to be reverted back to @import in commit 65163f9.

Once #199 is merged and Pages source is switched to GitHub Actions in repo settings, production will use Dart Sass and these changes are safe to make.

Tasks

• Restore @use / @forward in assets/main.scss (reverting the workaround from 65163f9)
• Replace any remaining @import rules in assets/sass/ with @use / @forward as appropriate (LibSass @import is deprecated in Dart Sass)
• Verify no deprecation warnings remain in bundle exec jekyll build output

Why this matters

• @import is deprecated in Dart Sass and will eventually be removed
• @use provides proper namespacing and avoids global variable leakage between partials
• Aligns local, Netlify preview, and production builds fully

Dependency

Must be done after #199 is merged and the Pages source is flipped to GitHub Actions in repo settings.