Update frontend PR workflow

Author: chrissearle

.github/workflows/pr-frontend.yaml

@@ -19,13 +19,17 @@ jobs:
     steps:
       - uses: actions/checkout@v6
 
+      - name: Set up pnpm
+        uses: pnpm/action-setup@v4
+        with:
+          package_json_file: frontend/package.json
+
       - name: Set up Node
         uses: actions/setup-node@v4
         with:
           node-version: "24"
-
-      - name: Set up pnpm
-        uses: pnpm/action-setup@v4
+          cache: "pnpm"
+          cache-dependency-path: frontend/pnpm-lock.yaml
 
       - name: Install dependencies
         run: pnpm install --frozen-lockfile