feat(auth): pass id_token_hint on Keycloak logout

Carries the raw ID token in the session cookie so logout can hand it back
to Keycloak as id_token_hint, letting it skip its own logout-confirmation
prompt -- otherwise a user could cancel out of it and land back in the app
with a Keycloak SSO session but no geniusrun profile (e.g. right after
deleting their account).

backend/internal/auth/mock/mock.go

@@ -18,6 +18,7 @@ type Verifier struct {
 	CallbackResult   auth.LoginResult
 	CallbackErr      error
 	EndSessionResult string // if empty, EndSessionURL returns postLogoutRedirectURL unchanged
+	LastIDTokenHint  string // records the idTokenHint passed to the last EndSessionURL call
 }
 
 var _ auth.Verifier = (*Verifier)(nil)
@@ -33,7 +34,8 @@ func (v *Verifier) HandleCallback(ctx context.Context, txn auth.TxnState, query
 	return v.CallbackResult, nil
 }
 
-func (v *Verifier) EndSessionURL(postLogoutRedirectURL string) string {
+func (v *Verifier) EndSessionURL(postLogoutRedirectURL, idTokenHint string) string {
+	v.LastIDTokenHint = idTokenHint
 	if v.EndSessionResult != "" {
 		return v.EndSessionResult
 	}