Wire trial_unification feature flag for .com Ultimate Trial signup

What does this MR do and why?

Wires the trial_unification feature flag to route the .com Ultimate Trial signup flow through the unified two-panel registration layout, and applies the new trial onboarding copy from Figma. Part of the Unified Registration: Trial epic.

This MR depends on the identity-verification marketing-panel delegation infrastructure landing separately (see !237925 (merged)). The trial-flow's identity-verification right-panel keeps the pipeline default once that lands.

References

• Rollout issue: gitlab-org/gitlab#601116
• Feature issue: gitlab-org&21601
• Depends on: !237925 (merged) (marketing-panel delegation infra)

Screenshots or screen recordings

Before (FF off — minimal 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 trial flow only unifies on SaaS):

   export GITLAB_SIMULATE_SAAS=1
   gdk restart

2. Enable the feature flag in the Rails console:

   Feature.enable(:trial_unification)

3. Sign out.
4. Visit the trial registration page: https://gdk.test:3443/-/trial_registrations/new
5. Verify (FF on):
   • The page renders in the two-panel layout with the marketing illustration on the right (.registration-marketing-panel-illustration).
   • The get-started subtext reads: "Start your free 30 day trial today, no credit card required. You'll have full access to our most advanced features, including GitLab Duo Agent Platform."
6. Sanity check (FF off): run Feature.disable(:trial_unification) in the console, reload /-/trial_registrations/new, and confirm it falls back to the single-panel minimal layout with no right-hand marketing panel.

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