fix(ci): build with pnpm instead of npm

package-lock.json was never updated after the switch to pnpm, so `npm ci`
failed on the outdated lock file. Use pnpm with the up-to-date
pnpm-lock.yaml, pin pnpm 11.26.0 via packageManager, move CI to Node 24
(required by pnpm 11) and drop the stale package-lock.json.

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

Author: generalentropy

.github/workflows/deploy.yml

@@ -22,16 +22,20 @@ jobs:
       - name: Checkout repository
         uses: actions/checkout@v4
 
+      - name: Setup pnpm
+        uses: pnpm/action-setup@v4
+
       - name: Setup Node
-        uses: actions/setup-node@v3
+        uses: actions/setup-node@v4
         with:
-          node-version: '18'
+          node-version: '24'
+          cache: 'pnpm'
 
       - name: Install dependencies
-        run: npm ci
+        run: pnpm install --frozen-lockfile
 
       - name: Build the site
-        run: npm run build
+        run: pnpm run build
 
       - name: Configure GitHub Pages
         uses: actions/configure-pages@v4