fix(deps): bump postcss to >=8.5.10 and patch brace-expansion (clear audit)

Resolves Dependabot GHSA-qx2v-qp2m-jg93 (postcss <8.5.10, CSS stringify XSS;
build-time devDependency) by raising the floor to ^8.5.10 (resolves to 8.5.15).
npm audit fix also patched a transitive brace-expansion ReDoS (GHSA-f886-m6hf-6m8v).

npm audit now reports 0 vulnerabilities. 174 unit tests pass, production build clean.

Author: kinncj

package-lock.json

@@ -30,7 +30,7 @@
     "@vitest/coverage-v8": "^4.1.1",
     "autoprefixer": "^10.4.17",
     "eslint": "^8.57.0",
-    "postcss": "^8.4.35",
+    "postcss": "^8.5.10",
     "tailwindcss": "^3.4.1",
     "vite": "^8.0.5",
     "vitest": "^4.1.1"