Add GitHub Pages PR preview deployment workflow

[Copilot]

This adds a dedicated pre-release GitHub Pages preview pipeline for pull requests so each new PR commit can publish an updated preview deployment. The existing production Pages workflow remains unchanged.

• What this introduces

  • New workflow: .github/workflows/github-pages-preview.yml
  • Triggered on PR lifecycle events that represent new reviewable code (opened, synchronize, reopened, ready_for_review)
  • Supports manual invocation via workflow_dispatch

• Preview deployment behavior

  • Builds with the repo-standard Pages command: npm run build:pages
  • Uploads the dist artifact with actions/upload-pages-artifact@v3
  • Deploys with actions/deploy-pages@v4 using preview: true to target the Pages preview channel

• Safety and CI ergonomics

  • Per-PR concurrency key to cancel stale in-flight preview deploys when newer commits arrive
  • Guard to avoid deploying previews from forked PRs (permissions/secrets-safe)
  • Minimal required permissions (contents: read, pages: write, id-token: write, pull-requests: write)

on:
  pull_request:
    branches: ["main"]
    types: [opened, synchronize, reopened, ready_for_review]

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

- name: Build
  run: npm run build:pages

- name: Deploy to GitHub Pages preview
  uses: actions/deploy-pages@v4
  with:
    preview: true

[Copilot]

Pull request overview

Adds a dedicated GitHub Pages preview deployment workflow for pull requests, enabling per-commit preview publishes while leaving the existing production Pages workflow unchanged.

Changes:

• Introduces a new PR-triggered workflow (pull_request + workflow_dispatch) to build and deploy Pages previews.
• Adds per-PR concurrency to cancel stale in-flight preview deployments on new commits.
• Guards deployment to avoid running preview deploys for forked PRs.