Add the credit stat panels to the DAP Impact Spend view

What does this MR do and why?

Adds the three credit stat panels to the DAP Impact Spend view: total credits with a previous-period trend, and the total and chat credit tiles. All on the DuoWorkflows aggregation engine; chat is the chat and agentic_chat/v1 flow types.

The panels set compact: true so large totals render as 1.45M once !255839 (merged) lands; until then the option is ignored and the stats show full digits, so this MR does not depend on it to merge.

Part of the per-panel Spend view MRs; panels take their final grid slots.

References

https://gitlab.com/groups/gitlab-org/-/work_items/23470

Screenshots or screen recordings

How to set up and validate locally

Enable the dap_impact_v1 feature flag, seed duo_workflows_workflows_enriched with credits data, then open /explore/analytics_dashboards/dap_impact?view=3.