The register flow exchanged the auth code as a confidential client
(Oidc:ClientId + Oidc:ClientSecret), requiring a pre-provisioned
backing app with a secret. Drop client_secret from the exchange so it
uses PKCE only — the backing app is now a public client and no secret
touches the client-tenant register/connect flows.
The toolbox's own OIDC sign-in still uses Oidc:ClientSecret (unchanged).
Also enable user-secrets (UserSecretsId) so Oidc config stays out of
the committed appsettings.json.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
kawa