chore: migrate from npm to pnpm

Replace package-lock.json with pnpm-lock.yaml, add packageManager field,
update GitHub Actions workflow to use pnpm/action-setup@v4, and convert
README instructional npm commands to pnpm equivalents.

.github/workflows/deploy.yml

@@ -19,12 +19,15 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
+      - uses: pnpm/action-setup@v4
+        with:
+          version: 11
       - uses: actions/setup-node@v4
         with:
           node-version: 20
-          cache: npm
-      - run: npm ci
-      - run: npm test
+          cache: 'pnpm'
+      - run: pnpm install --frozen-lockfile
+      - run: pnpm test
 
   build-deploy:
     needs: test
@@ -34,12 +37,15 @@ jobs:
       url: ${{ steps.deployment.outputs.page_url }}
     steps:
       - uses: actions/checkout@v4
+      - uses: pnpm/action-setup@v4
+        with:
+          version: 11
       - uses: actions/setup-node@v4
         with:
           node-version: 20
-          cache: npm
-      - run: npm ci
-      - run: npm run build-nolog
+          cache: 'pnpm'
+      - run: pnpm install --frozen-lockfile
+      - run: pnpm build-nolog
       - uses: actions/configure-pages@v5
       - uses: actions/upload-pages-artifact@v3
         with: