Add concurrency groups to CI workflows

[gkorland]

Parent issue

Part of #550 (CI Pipeline Optimization Analysis)

Problem

Neither the Build (nextjs.yml) nor Playwright Tests (playwright.yml) workflows have concurrency groups. Rapid pushes to the same branch or PR queue redundant runs instead of cancelling superseded ones.

Solution

Add concurrency groups to both workflows:

concurrency:
  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
  cancel-in-progress: true

Estimated savings

Variable — prevents wasted minutes from stacked redundant runs

Effort

Trivial