Add npm cache to CI workflows

[gkorland]

Parent issue

Part of #550 (CI Pipeline Optimization Analysis)

Problem

Neither workflow caches npm dependencies. Every run downloads all npm packages from scratch.

Solution

Add cache: 'npm' to the setup-node action:

- uses: actions/setup-node@v4
  with:
    node-version: 24
    cache: 'npm'
    cache-dependency-path: |
      package-lock.json
      app/package-lock.json

Apply to both nextjs.yml and playwright.yml.

Estimated savings

~3-5s per job (~10s total)

Effort

Trivial