Wire free_registration_unification feature flag for .com free signup

What does this MR do and why?

Wires the free_registration_unification feature flag to route the .com Free signup flow through the unified two-panel registration layout. Part of the Unified Registration: Free epic.

The identity-verification marketing-panel delegation infrastructure (!237925 (merged)) has now merged, so the free flow's identity-verification right-panel image and heading switch to subscription/collab + "Collaborate and accelerate in one place" alongside the two-panel layout switch.

References

• Rollout issue: gitlab-org/gitlab#601119
• Feature issue: gitlab-org&21602
• Built on: !237925 (merged) (marketing-panel delegation infra — merged)
• Companion MR: !237932 (merged) (trial unification — merged)

Screenshots or screen recordings

Before (FF off — devise single-panel)	After (FF on — registration_two_panel)

How to set up and validate locally

1. Run GDK in SaaS mode so Onboarding.enabled? is true (the free flow only unifies on SaaS):

   export GITLAB_SIMULATE_SAAS=1
   gdk restart

2. Enable the feature flag in the Rails console:

   Feature.enable(:free_registration_unification)

3. Sign out.
4. Visit the standard registration page: https://gdk.test:3443/users/sign_up
5. Verify (FF on):
   • The page renders in the two-panel layout ([data-testid="registration-two-panel-layout"]) with the marketing illustration on the right.
6. Sanity check (FF off): run Feature.disable(:free_registration_unification) in the console, reload /users/sign_up, and confirm it falls back to the single-panel devise layout (.signup-page) with no right-hand marketing panel.

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