Add a build check for pull requests

The deploy workflow only runs on pushes to main, so a pull request got no
build status and a broken build showed up only after merging. A separate
Build workflow now runs `pnpm build` on every pull request, with a
read-only token. pages.yml is unchanged, so deploys keep their own
permissions.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: alekswebnet

.github/workflows/build.yml

@@ -0,0 +1,26 @@
+# Builds pull requests so a broken build shows up before merging.
+# Deploys stay in pages.yml, which runs on pushes to main.
+name: Build
+
+on:
+  pull_request:
+
+permissions:
+  contents: read
+
+concurrency:
+  group: build-${{ github.event.pull_request.number }}
+  cancel-in-progress: true
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v7
+      - uses: pnpm/action-setup@v6
+      - uses: actions/setup-node@v7
+        with:
+          node-version: 24
+          cache: pnpm
+      - run: pnpm install --frozen-lockfile
+      - run: pnpm build

README.md

@@ -17,7 +17,7 @@ pnpm dev     # http://localhost:8080
 pnpm build   # writes _site/
 ```
 
-Pushes to `main` deploy through [.github/workflows/pages.yml](.github/workflows/pages.yml).
+Pushes to `main` deploy through [.github/workflows/pages.yml](.github/workflows/pages.yml). Pull requests get a build check from [.github/workflows/build.yml](.github/workflows/build.yml).
 
 ## Editing the pdfjs-viewer docs