Add configurable OIDC login and Atlas-style shell

Summary

• Add a generic, env-configured OIDC login flow with PKCE, session storage, and logout support.
• Copy the Atlas-style Noora login screen, headerbar, sidebar, and account control structure while keeping product and provider labels configurable.
• Extend runtime config, README docs, and the Helm chart for generic auth branding and optional OIDC secrets.
• Add focused controller coverage for the auth gate and login page copy.

Testing

• mix format --check-formatted
• mix test
• mix assets.build
• helm template hive infra/helm/hive --set image.tag=test
• PORT=4861 mix phx.server with curl checks for / and /login

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