feat(oidc): add admin UI for OIDC configuration

Phase 4 implementation of OIDC authentication support.

Backend API endpoints:
- GET /skServer/security/oidc - retrieve OIDC config (secrets redacted)
- PUT /skServer/security/oidc - update OIDC config in security.json
- POST /skServer/security/oidc/test - test connection via discovery doc

Frontend UI:
- OIDCSettings.js - new collapsible card for OIDC configuration
- Settings.js - integrate OIDCSettings component
- Users.js - show SSO/Local badge and OIDC user email, hide password
  fields for SSO users

Features:
- Environment variable overrides shown as disabled fields with ENV badge
- Client secret preserved when not provided in update
- Test Connection button to verify OIDC provider discovery
- isOIDC indicator in user list API response

Refs: #5

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: mairas