fix: point pnpm setup action at app/package.json

The pnpm/action-setup@v4 action looks for packageManager in the repo
root by default. Since our package.json lives in app/, we need to
specify package_json_file explicitly.

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

Author: dakom

.github/workflows/tauri.yml

@@ -35,6 +35,7 @@ jobs:
       - name: Install pnpm
         uses: pnpm/action-setup@v4
         with:
+          package_json_file: app/package.json
           run_install: false
 
       - name: Install frontend dependencies