PR-3: Add E2E coverage and finalize i18n for project admin

**Depends on PR-0, PR-1a, PR-1b, PR-2a, PR-2c, PR-2d (PR-2b nice-to-have).** Final verification step.

## Scope

- Playwright E2E tests under `e2e/` using project-admin account on test server:
- Endpoint: `http://10.122.10.215:8090`
- User: `project-admin-for-default@lablup.com`
- Project: `default`
- Test scenarios:
1. Login as project admin → Admin menu category visible.
1. Each Admin page (Session / Serving / VFolder / Members) shows only `default` project data.
1. ProjectSelect shows 'Project Admin' badge on `default` project.
1. Switching to a non-admin project in admin mode triggers confirm dialog; accept exits admin mode.
1. General-user pages still show only own resources.
- i18n keys for all new strings (badge text, confirm dialog message, page titles, column headers) — invoke `i18n-translator` agent or run `/fw:i18n` to fill in all target languages.
- `bash scripts/verify.sh` clean across the entire stack.
- `/fw:batch-review` pass against the stack.

## Acceptance

- All E2E scenarios pass on test server.
- All locale files have translations for new keys.
- `scripts/verify.sh` ends with `=== ALL PASS ===` on every PR in the stack.
- batch-review surfaces no critical / high severity findings.

## Reference

- Test credentials saved in agent memory: `reference_e2e_project_admin.md`
- Spec: `.specs/FR-2209-project-admin-management/spec.md`
- Dev plan: `.specs/FR-2209-project-admin-management/dev-plan.md` (PR-3 section)

JIRA Issue: FR-2558