Add frontend unit tests for WorkflowSnapshotService

### Task Summary
The frontend service at `frontend/src/app/dashboard/service/user/workflow-snapshot/workflow-snapshot.service.ts` currently has no unit-test coverage. Add a spec covering the canvas capture (via html2canvas), the multipart upload (FormData with wid handling) and the retrieve endpoint, exercising both success and error paths.
### Task Type
- [x] Testing / QA

Start with frontend/src/app/dashboard/service/user/workflow-snapshot/workflow-snapshot.service.ts and trace the canvas capture, multipart upload, and retrieve endpoint. The work is done when a spec exercises success and error paths for html2canvas capture, FormData upload with wid handling, and retrieval.

Written by the indexing model from the issue text.