Add SCSS unit tests for mixins, utilities, and helpers (#2699)

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

core/vitest.scss.config.mjs

@@ -0,0 +1,10 @@
+import { defineConfig } from 'vitest/config'
+
+// Standalone project for SCSS unit tests (sass-true).
+// Runs in the default Node environment — intentionally separate from the
+// browser-mode JS/TS config in vitest.config.mts so the two never overlap.
+export default defineConfig({
+  test: {
+    include: ['scss/tests/**/*.test.mjs'],
+  },
+})