feat(oauth): support desktop-specific OIDC issuer and client_id discovery

[pat-s]

fix #246

Enables identity providers that require separate OIDC clients per application type (like Authentik, Kanidm, Zitadel) to work with the desktop client.

• Discover desktop-specific OIDC issuer via WebFinger (http://openid.net/specs/connect/1.0/issuer/desktop)
• Use server-provided client_id from WebFinger link properties when available
• Persist and restore client_id across re-authentication

Server-side changes

To work, this PR requires changes in https://github.com/opencloud-eu/opencloud.
A complementary PR is available at opencloud-eu/opencloud#2072.

Additionally, the following env vars must be set on the admin side:

WEBFINGER_OIDC_ISSUER_DESKTOP: <>
WEBFINGER_OIDC_CLIENT_ID_DESKTOP: <>

Here, WEBFINGER_OIDC_ISSUER_DESKTOP is usually the root URL of the identity provider and WEBFINGER_OIDC_CLIENT_ID_DESKTOP contains the client_id of the respective oAuth project.

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With these changes, I was able to login into the Desktop app with a dedicated oAuth project through Zitadel (v4.7.0). The implementation should be generic and work with other oAuth providers as well.