Add Agentic Vuln Management adoption funnel panel

What does this MR do?

Introduces the "SAST triage and remediation funnel" panel on the group and project security dashboards, gated behind the new security_dashboard_agentic_adoption wip feature flag (disabled by default).

Actual querying is done in next iterations.

Related to #600940 (closed) #600941 (closed)

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. With the flag off (default), open the group and project security dashboards and confirm the "SAST triage and remediation funnel" panel is not shown.
2. Enable the flag: Feature.enable(:security_dashboard_agentic_adoption)
3. Reload both dashboards and confirm:
   • A full-width panel titled "SAST triage and remediation funnel" appears on its own row, below the existing panels.
   • The panel header shows a time-period selector (30 / 60 / 90 days), and changing it updates the selection with no console errors.

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