refactor(web): extract POST_LOGIN_REDIRECT_KEY constant

[thxforall]

What to build

The sessionStorage key post_login_redirect is a magic string duplicated across ~8 production files (OnboardingFlow, LoginCard, SessionExpiredBanner, AuthGate, and the Cody screens ItemDetail / SavedLooks / LookDetail / RecommendedLooks) and read centrally in authStore.ts.

Extract a shared constant POST_LOGIN_REDIRECT_KEY and use it at every producer and the consumer.

Scope constraint: this is a constant extraction only. Do NOT bundle the auth-check / write / routing behavior into a module — the consumer-side orchestration in authStore stays where it is.

Acceptance criteria

• POST_LOGIN_REDIRECT_KEY constant defined once and imported by all producers + authStore
• No literal "post_login_redirect" remains in production code (grep clean; tests may keep literals or adopt the constant)
• Login-redirect flow behavior is unchanged; tests pass

Blocked by

None - can start immediately

[thxforall]

Closed automatically because PR #720 was merged into dev: #720