fix: run backend coverage from repo root, not frontend dir

- Remove working-directory: frontend from coverage step
- cd back to repo root for ./gradlew command, then cd frontend for npm
- Gradle wrapper lives at repo root, not in frontend/

.forgejo/workflows/ci.yml

@@ -50,9 +50,10 @@ jobs:
 
       - name: Coverage verification
         run: |
+          cd ..
           ./gradlew :backend:jacocoTestCoverageVerification
+          cd frontend
           npm run test:coverage
-        working-directory: frontend
 
   e2e:
     name: E2E browser tests