[dashboard] Fix logout (#1510)

Signed-off-by: Andrei Kvapil <kvapss@gmail.com>

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## What this PR does


### Release note

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```release-note
[dashboard] Fix logout
```

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## Summary by CodeRabbit

* **New Features**
* Enhanced OIDC logout flow: backend logout is now supported, improving
reliability of signing out across services.
* Whitelisted the identity provider domain to enable seamless redirects
during authentication and logout journeys.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

packages/system/dashboard/templates/gatekeeper.yaml

@@ -55,6 +55,8 @@ spec:
             - --http-address=0.0.0.0:8000
             - --redirect-url=https://dashboard.{{ $host }}/oauth2/callback
             - --oidc-issuer-url=https://keycloak.{{ $host }}/realms/cozy
+            - --backend-logout-url=https://keycloak.{{ $host }}/realms/cozy/protocol/openid-connect/logout?id_token_hint={id_token}
+            - --whitelist-domain=keycloak.{{ $host }}
             - --email-domain=*
             - --pass-access-token=true
             - --pass-authorization-header=true