fix(accounts): expose OAuth device code in official login flow

[GcsSloop]

Summary

• add backend device auth call to fetch and return OAuth user code
• include device auth metadata in /accounts/auth/authorize response
• show device code and copy action in official account OAuth modal
• add backend and frontend tests for device code flow

Verification

• go test ./internal/auth ./internal/api
• pnpm -C frontend test -- AccountsPage.test.tsx