Add usage trends chart to usage app

What does this MR do and why?

The MR adds the usage trends chart to the usage app.

note: the feature is behind a feature flag, it won't be enabled until the whole project is ready.

MR	status
Add usage billing app skeleton with its story (!203124 - merged)	✅
Add formatted-date-range component to usage_bil... (!203275 - closed)	⏳
Add usage trends chart to usage app (!203360 - merged)	this MR

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

• Ensure you're running GitLab as self-managed (e.g. 'export GITLAB_SIMULATE_SAAS=0 gdk restart`)
• Ensure you've a license added to your instance
• Enable billing_usage feature flag (e.g. http://gdk.test:3000/rails/features/usage_billing_dev)
• Navigate to the /admin/gitlab_duo/usage
• Go to "Usage by user" tab
• Verify the usage trends chart is rendering

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #566204