Apply the prototype's layout to the policy editor

What does this MR do and why?

This merge request refreshes the visual design of the policy editor interface. Key changes include:

• The policy name and description fields are extracted into their own reusable component, making the header cleaner and easier to maintain.
• Step navigation controls and the enforcement mode selector are moved into a dedicated controls component, simplifying the main wizard layout.
• The enforcement mode options (Enforce, Warn, Audit) now include descriptive text explaining what each mode does, and their icons are updated to clearer status-style icons.
• Various styling tweaks improve spacing, typography, and visual consistency throughout — for example, borders are more subtle, text weights are adjusted, and icon containers are given a rounded background.
• The expand/collapse toggle on policy entries is removed, so configuration fields are always visible rather than hidden behind a button click.
• The bottom action bar (Cancel/Back/Next buttons) is now sticky, staying visible at the bottom of the screen as the user scrolls through the policy canvas.
• Segment button selection now uses the standard library "selected" treatment instead of custom border styling.

Fourth MR of the chain from https://gitlab.com/gitlab-org/gitlab/-/issues/607341:

1. !248392 (merged) — catalog categories
2. !248394 (merged) — building blocks
3. !247891 (merged) — the Build policy step (this MR's target; retargets to master as the chain merges)
4. This MR — prototype styling

References

• Plan: https://gitlab.com/gitlab-org/gitlab/-/issues/607341

Screenshots or screen recordings

UI change — screenshots required. TODO before marking ready:

Before	After

How to set up and validate locally

The Policy Store is behind a flag and a two-level experiment opt-in:

1. Enable the feature flag:

   Feature.enable(:security_policies_v2)

2. Enable the instance setting:

   ApplicationSetting.current.update!(policy_store_experiment_enabled: true)

3. Enable it for a top-level group with an Ultimate licence:

   Group.find_by_full_path('your-group').namespace_settings.update!(policy_store_experiment_enabled: true)

4. Go to Group → Secure → Policy Store, then New policy.

5. On Build policy: pick the Deployment trigger, then a rule or two, then an action. Check that

   • selecting a second trigger replaces the first
   • each selected entry expands into its own config form
   • the Rego rule is pre-filled with the deployment gate template and Browse templates works
   • values survive clicking Next to Scope and Back again

yarn jest ee/spec/frontend/policy_store

MR acceptance checklist

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