Define sToken login boundary component spec

[nowgnuesLee]

Purpose

Author and land the feature spec for the reusable STokenLoginBoundary React component tracked by the parent Epic FR-2616.

Deliverable

• Korean spec at `.specs/draft-stoken-login-boundary/spec.md`
• Metadata at `.specs/draft-stoken-login-boundary/metadata.json`
• Spec PR: #6828

Scope

Spec-only; no production code (LoginView.tsx, EduAppLauncher.tsx) is modified in this task. Actual implementation and migrations are tracked as sibling stories under the Epic, to be created via `/dev-plan` after this spec lands.

Done When

• PR feat(FR-2618): add sToken login boundary component spec #6828 is reviewed and merged.
• `metadata.json` `specTaskKey` populated with this issue's key.
• Directory renamed from `draft-stoken-login-boundary` to `FR-2616-stoken-login-boundary`.
• Epic FR-2616 has the spec PR as a web link (already done).

JIRA Issue: FR-2618