getAuthenticatedUser only read ID token claims, where `name` is often
absent, so the web app showed the user's email instead of their name
after SSO. Fetch the userInfo endpoint and resolve the display name with
the same fallback order as the vitrine (la-compagnie-maximus#80):
userInfo.name -> userInfo.username -> claims.name -> claims.username,
with the email fallback applied in the layout. Email now also prefers
userInfo over claims.
Single source of truth in auth.ts (only producer of `name`, consumed
solely by layout.tsx -> Header). layout.tsx and Header.tsx verified,
left unchanged.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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