Disable DAP flow toggles in project settings when platform is off

What does this MR do and why?

When duo_agent_platform_enabled is disabled at the instance level, the "Allow flow execution" and "Allow foundational flows" toggles in Project > Settings > GitLab Duo remained visible and interactive. They should be disabled (but still visible) when DAP is off at the instance level.

This fix adds a duoAgentPlatformEnabled prop to GitlabDuoSettings, populated from Ai::DuoWorkflow.duo_agent_platform_available?, and includes it in the :disabled condition of both flow toggles in the non-readiness-card path. The readiness-card path already handled this correctly via platformEnabled.

Sub-setting values are preserved in the database — only the interactability of the UI controls is affected, as intended.

References

#596171 (closed)

Screenshots or screen recordings

Before	After
Flow toggles remain interactive when DAP is disabled at instance level	Flow toggles are disabled when DAP is disabled at instance level

How to set up and validate locally

1. In the Rails console, disable DAP for your environment.

   For a SaaS GDK:

   root_group = Group.find_by_full_path('your-group').root_ancestor
   root_group.ai_settings.update!(duo_agent_platform_enabled: false)

   For a self-managed or Dedicated GDK:

   organization = Organizations::Organization.default_organization
   Ai::Setting.for_organization(organization).update!(duo_agent_platform_enabled: false)

2. Visit any project under the selected group or organization, then open Settings > General > GitLab Duo.

3. Verify that the "Allow flow execution" and "Allow foundational flows" toggles are visible but disabled.

4. Re-enable DAP using the applicable command above with true and verify the toggles become interactive again.

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