fix(ci): build OIDC app for all test modes including single-user

The OIDC submodule volume mount is on the base app service, so all
modes mount it. Without composer install, the post-install hook enables
a broken app (missing vendor/autoload.php), causing Nextcloud to fail.

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

.github/workflows/test.yml

@@ -123,7 +123,6 @@ jobs:
           npm run build
 
       - name: Build OIDC app
-        if: matrix.mode != 'single-user'
         run: |
           cd third_party/oidc
           composer install --no-dev --optimize-autoloader