feat: #1007 - Option to disable local login when OIDC is enabled

[DerDummePunkt]

This PR attempts to implement #1007

Adds configurable local authentication through GOTIFY_LOCALAUTH_ENABLED, defaults to true to preserve existing behavior.

When local authentication is disabled:

• The server does not register POST /auth/local/login.
• Basic Auth with local Gotify user credentials is rejected by authentication middleware.
• The check for a local admin account and subsequent creation is skipped.
• The UI hides the local username/password login form, registration button, and password-based elevation form.
• The UI receives the public localauth flag through injected window.config.
• GET /gotifyinfo exposes the same flag as localauth.

The server exits with a fatal error when both local authentication and OIDC are disabled, preventing startup without an available login method.