fix: skip postinstall webpack build in test workflow

The postinstall script runs webpack which fails due to TypeScript
version mismatch with @types/node. The test workflow only needs
vitest, not the frontend bundle.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: mlahargou

.github/workflows/test.yml

@@ -16,7 +16,7 @@ jobs:
           node-version: "24"
           cache: npm
 
-      - run: npm install
+      - run: npm install --ignore-scripts
 
       - name: Test
         run: npm test