ci: install pnpm globally for Chromatic workflow

Chromatic action runs `pnpm run build-storybook` internally, which
fails with ENOENT when pnpm is not available. Add `vp install -g pnpm`
after setup-vp to ensure pnpm is on PATH.

Author: fengmk2

.github/workflows/chromatic.yml

@@ -31,6 +31,9 @@ jobs:
           node-version: lts/*
           cache: true
 
+      - name: 🟧 Install pnpm globally
+        run: vp install -g pnpm
+
       - name: 🧪 Run Chromatic Visual and Accessibility Tests
         uses: chromaui/action@f191a0224b10e1a38b2091cefb7b7a2337009116 # v16.0.0
         env: