Add Naver and Kakao social login

[ICubE01]

Summary

• Add Naver (OAuth2) and Kakao (OIDC) as social login providers alongside the existing Google login
• Unify all three OAuth callback pages into a single SocialCallbackPage component
• Make social login the primary login method on the login page; email/password is now a collapsible secondary option
• Translate backend social auth error messages to Korean on the frontend

Backend

Features

• KakaoOidcProviderClient — Kakao OIDC provider with nonce verification and email validation
• NaverOAuth2ProviderClient — Naver OAuth2 provider with manual token exchange and user-info fetch
• SocialAuthUserInfo — common user info record decoupling providers from OidcIdToken
• SocialAuthProviderName — renamed/extended from OidcProviderName to cover all three providers

Enhancements

• GoogleOidcProviderClient, KakaoOidcProviderClient, NaverOAuth2ProviderClient — validate subject and email are non-null before constructing SocialAuthUserInfo

Frontend

Features

• SocialCallbackPage — single callback component replacing three separate pages; accepts SocialAuthProviderName prop
• socialAuthErrors.ts — maps known English backend error messages to Korean, with fallback to original message
• LoginPage — added Naver and Kakao buttons; email/password form is now hidden behind a collapsible toggle

Enhancements

• Callback pages read the response body on failure and display the backend ErrorDto.message (translated to Korean where possible)
• ErrorDto type added to types.ts