add gitlab as an sso provider

[masnwilliams]

Summary

Adds GitLab to the list of recognized SSO providers in getSSOProviderInfo, so any provider key containing gitlab renders with the GitLab label and tanuki mark.

• New GitLabMark SVG icon in icons.tsx.
• Mapping entry for gitlab in sso-provider.tsx.

Test plan

• bun run typecheck
• bun run --filter '@onkernel/managed-auth-react' build
• Verify in demo that a gitlab provider key renders the correct label and icon

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Note

Low Risk
Low risk UI-only change: adds a new SVG icon and a simple gitlab string match in getSSOProviderInfo, with no auth flow or data-handling logic modified.

Overview
Adds GitLab as a recognized SSO provider in managed-auth-react so provider keys containing gitlab render with a GitLab label and the new GitLabMark icon.

Introduces the GitLabMark SVG in icons.tsx and wires it into getSSOProviderInfo in sso-provider.tsx.

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