Add WG_IDP_CONFIG_FILE env var to seed OIDC/SAML identity providers
from a YAML file at startup, enabling GitOps and IaC workflows.
Providers are upserted by id (merge strategy preserves manual additions).
Convert all e2e tests from NiceGUI User fixture to Playwright async API
with --headed and --slowmo flags for visual debugging. Add full OIDC
login flow test against the mock-oidc service.
