refactor(WebUI Smoke CLI): extract single-account-safe e2e utilities

## Parent

- Epic: [FR-2871](https://lablup.atlassian.net/browse/FR-2871)
- Spec Task: [FR-2872](https://lablup.atlassian.net/browse/FR-2872)
- Spec: `.specs/FR-2871-webui-smoke-cli/spec.md` § FR-F

## Goal

Audit `e2e/utils/test-util.ts` for assumptions requiring multiple seeded accounts (admin / user / user2 / monitor) and extract single-account-safe helpers into a separate module that smoke specs use exclusively. Ensures the smoke catalog cannot accidentally depend on a multi-account fixture.

## Acceptance criteria

- [ ] Audit `e2e/utils/test-util.ts` for assumptions requiring multiple seeded accounts (admin, user, user2, monitor).
- [ ] Extract single-account-safe helpers into a separate module that smoke specs use.
- [ ] No smoke spec may import a helper that requires more than one account.
- [ ] Smoke specs run green against a fixture environment provisioned with exactly one account.

## In scope

- `e2e/utils/test-util.ts` (audit, no behavioural change to non-smoke callers)
- `e2e/utils/smoke-util.ts` (new, single-account helpers only)
- `e2e/tests/**` — smoke-tagged specs only, redirected to the new helpers as needed

## Out of scope

- Authoring new smoke specs (FR-J)
- Removing multi-account helpers used by non-smoke specs

## Test plan

- Provision a fixture environment with exactly one user account; run `@smoke` selection green.
- Run the full e2e suite with the existing multi-account fixture; confirm no regression.

JIRA Issue: FR-2880

[FR-2871]: https://lablup.atlassian.net/browse/FR-2871?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ
[FR-2872]: https://lablup.atlassian.net/browse/FR-2872?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ