Logto requires PKCE (Proof Key for Code Exchange) for SPA auth.
Added code_challenge/code_verifier to login and callback flow.
Also fixed Traefik router-service linking — when a container defines
multiple routers, each needs an explicit service binding or Traefik
v3 refuses to auto-link them.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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