e2e: E2E login suite failing on deployed endpoint (all auth/login specs + cleanup teardown)

## Summary
Scheduled/dispatched weekday E2E run failed with `TEST_EXIT_CODE=1`. **All** specs in `e2e/auth/login.spec.ts` failed, plus both `global-cleanup.teardown.ts` cases (which rely on an admin login to clean up leftover vfolders/services).

- **Commit SHA**: `d2f20c9ed96736eee214350ca0604764789de1d4`
- **Workflow run**: https://github.com/lablup/backend.ai-webui/actions/runs/34004668589
- **Env source**: `E2E_WEBUI_ENDPOINT` = `(redacted)` (deployed Amplify endpoint)

## Failing cases
| Spec | Test | Cause analysis |
|---|---|---|
| `e2e/auth/login.spec.ts` | `should display the login form` | Server/config |
| `e2e/auth/login.spec.ts` | `should redirect to the Summary` | Server/config |
| `e2e/auth/login.spec.ts` | `user can login with endpoint that has a single trailing slash` | Server/config |
| `e2e/auth/login.spec.ts` | `user can login with endpoint that has multiple trailing slashes` | Server/config |
| `e2e/auth/login.spec.ts` | `API requests do not contain double-slash after endpoint normalization` | Server/config |
| `e2e/auth/login.spec.ts` | `should display error message for non-existent email` | Server/config |
| `e2e/auth/login.spec.ts` | `should display error message for incorrect password` | Server/config |
| `e2e/global-cleanup.teardown.ts` | cleanup leftover e2e vfolders (user) | Server/config (downstream of login) |
| `e2e/global-cleanup.teardown.ts` | cleanup folders and services (admin) | Server/config (downstream of login) |

## Cause analysis
The failure spans the **entire** login suite, including the most basic case (`should display the login form`, which only asserts the page renders — no auth call involved). Checked the last 3 commits reachable in this checkout (`0ff2215`…`d2f20c9`, PR #9446 and the current `chore/pnpm-12` branch): none touch `react/src` auth/login components, i18n strings, or anything reachable from the login page — the diff is entirely pnpm 12 tooling migration (`package.json`, lockfile, CI actions, patches). That rules out a recent WebUI code change as the cause.

Given uniform failure from page render through redirect/error-message assertions, and that cleanup (which also logs in) failed identically, this looks like a **Server/config issue** — most likely the deployed endpoint (`E2E_WEBUI_ENDPOINT`) was unreachable, slow, or returning an unexpected response at run time — rather than a Backend API contract change or a targeted WebUI regression. This sandbox's network egress is restricted, so the endpoint could not be curl-checked or the recorded Playwright traces (`trace.zip`) programmatically inspected (`unzip`/`python`/`node`/`curl` all denied) to confirm the exact error (timeout vs. connection refused vs. unexpected DOM). Manual inspection of the traces or the Amplify app's build/deploy status is recommended as the next step.

## Related PRs
- #9446 — feat(FR-3861): mark list links with the access their WebUI page requires — unrelated to auth/login, no overlap with failing specs.
- Current branch `chore/pnpm-12` (PR #9447) — dependency/tooling migration only, unrelated to auth/login.

## Repro
```bash
pnpm --filter e2e exec playwright test e2e/auth/login.spec.ts --project=chromium
```
(requires `E2E_WEBUI_ENDPOINT` and the configured E2E test account env vars — not reproduced with credentials in this report)

## Artifacts
- `playwright-report/` and per-test `trace.zip` files are available in the workflow run's uploaded artifacts for the 9 failing tests listed above.

> AI generated by [E2E Watchdog & Healer](https://github.com/lablup/backend.ai-webui/actions/runs/34004668589) for #9447

Start with e2e/auth/login.spec.ts and e2e/global-cleanup.teardown.ts, then reproduce with the provided Playwright command and configured E2E environment variables. Inspect the workflow's playwright-report/ and trace.zip artifacts and check the deployed Amplify endpoint and build status. Done means identifying and addressing the server or configuration cause so the login and cleanup cases pass.

Written by the indexing model from the issue text.