Add two-panel registration layout for SM subscription flow

What does this MR do and why?

Introduces a feature-flagged two-panel registration layout for /users/sign_up as part of the Unified Registration: Subscription on Self-Managed initiative.

When subscription_sm_unification (gitlab_com_derisk) is enabled, the page renders a left form panel with SM-specific copy and a right marketing panel. When disabled, the existing layout is preserved unchanged.

References

• Epic: https://gitlab.com/groups/gitlab-org/-/epics/21432
• Work item: https://gitlab.com/gitlab-org/gitlab/-/work_items/594274

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Enable the feature flag in rails console:

   Feature.enable(:subscription_sm_unification)

2. Visit /users/sign_up
3. Verify the two-panel layout renders with the marketing panel on the right
4. Disable the flag and verify the original layout is restored:

   Feature.disable(:subscription_sm_unification)

MR acceptance checklist

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