feat: implement OAuth 2.0 authorization flow with TimeHarbor integration

[Dharp02]

This pull request adds support for acting as an OpenID Connect (OIDC) provider, enabling OAuth 2.0 Authorization Code flows for integration with trusted clients like TimeHarbor. It introduces the OIDC provider plugin, configures trusted client details, and ensures the correct handling of URL-encoded bodies required by OIDC endpoints. Additionally, it updates the frontend to support seamless OAuth flows by forwarding authorization parameters and session tokens as needed.

OIDC Provider Integration and Configuration:

• Added the oidcProvider plugin to the betterAuth setup in auth.ts, registering TimeHarbor as a trusted OAuth client and configuring the login page, client secrets, and consent skipping.

Backend Request Handling Improvements:

• Updated server.ts to accept application/x-www-form-urlencoded bodies (used by OIDC token endpoints) and forward them as raw strings to the authentication handler, ensuring compatibility with OIDC flows.
  Frontend OAuth Flow Support:

• Modified main.tsx to detect OAuth 2.0 authorization parameters in the URL and, if the user is authenticated, forward them (along with the session token in a cookie) to the OIDC authorization endpoint.

• Updated LoginForm.tsx so that after a successful sign-in initiated by an OAuth 2.0 authorization request, the app redirects back to the authorization endpoint, copying the session token into a cookie for Better Auth to recognize the session.