Support non-OIDC auth providers

## Background

Currently, the STAC Manager makes use of the Keycloak JS module for authentication and keycloak-specific environment variables for configuration:

https://github.com/developmentseed/stac-manager/blob/ab0a5cef42b60daddabe6a056757b25290bc2dc5/packages/client/src/auth/Context.tsx#L8-L22

However, there's nothing Keycloak specific about our usage of auth. We do make a call to the keycloak `/account` endpoint:
https://github.com/developmentseed/stac-manager/blob/ab0a5cef42b60daddabe6a056757b25290bc2dc5/packages/client/src/auth/Context.tsx#L68-L69

However, the data returned by that endpoint overlaps with the data returned in the `idToken` when a user fetches their auth token.

## Goals

Refactor our auth tooling to support an OIDC provider.

Start with packages/client/src/auth/Context.tsx, especially the Keycloak setup at lines 8–22 and the /account call at lines 68–69. Compare the account response with the idToken data described in the issue, then trace the related Keycloak-specific configuration. Done means the auth tooling supports an OIDC provider without depending on Keycloak-specific authentication data.

Written by the indexing model from the issue text.