Add enhanced authentication to support additional auth flows

[dustins]

Summary
This PR introduces a Backend for Frontend (BFF) authentication architecture that abstracts IdP integration from the frontend SPA, addressing a key integration pain point for current and prospective customers.

Problem: MLSpace currently supports only PKCE-based OIDC authentication, preventing integration with IdPs that exclusively use OIDC flows requiring backend secrets. This limitation has created friction for customers and blocks future integrations with SAML-only IdPs.

Solution
The new BFF pattern moves authentication logic to the backend, creating an extensible architecture that:

• Provides a simplified, protocol-agnostic authentication API for the frontend
• Enables support for secret-based OIDC flows
• Maintains compatibility with existing PKCE authentication
• Establishes a foundation for future IdP integrations

Changes include:

• Backend API modifications to handle authentication flows server-side
• Frontend updates to consume the new abstracted authentication API
• New authentication flow connecting frontend and backend components

This enhancement removes IdP and protocol-specific dependencies from the frontend, significantly improving MLSpace's flexibility for customer integrations.

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[dustins]

initial review complete

[dustins]

completed second review