refactor: use named export for UnauthorizedForm

What & Why

UnauthorizedForm was the only component in components/ using a default export — every other component uses a named export. Switched it to a named export and updated the import in app/unauthorized/page.tsx to match the convention.

Also documented in AGENTS.md:

• the export convention (components/ → named; Next.js file conventions follow the framework's required shape).
• how the /unauthorized page resolves its locale (it lives outside app/[lang]/, so it reads Accept-Language).

Related Issue

N/A

How to Verify

1. npm run typecheck && npm run lint && npm run build — all pass.
2. Visit /unauthorized → the token form still renders, localized via Accept-Language, and submitting a token works. (No behavior change — export refactor only.)

Checklist

• PR title follows Conventional Commits (feat:, fix:, chore:, refactor:, docs:, i18n:)
• Translations added to both locales/ko.json and locales/en.json (if UI text changed)
• Tested on mobile viewport (if UI changed)