Fix npm vulnerabilities with available updates (postcss remains unresolved)

During a recent `npm audit`, several security vulnerabilities were detected in this project. This PR addresses all vulnerabilities for which fixes are currently available.

**Vulnerabilities addressed:**

* `@eslint/plugin-kit` ✅
* `brace-expansion` ✅
* `next` ✅

**Vulnerabilities remaining:**

* `postcss` ⚠️ (moderate severity; no fix available yet)



