Add net change tooltip

[prschulz]

Got some code for us? Awesome 🎊!

Please take a minute to explain the change you're making:

• Why are you making it?
  To address Linear issue NY-863, which highlighted that publishers need to manually calculate the net change in paid members from the "New" and "Cancelled" values in the growth chart tooltip.
• What does it do?
  This PR adds a "Net change" line to the tooltip on the paid members change chart. It calculates the sum of "New" and "Cancelled" members for the period and displays it with appropriate + or - formatting.
• Why is this something Ghost users or developers need?
  Ghost users (publishers) will gain immediate insight into their net subscriber growth or loss without needing to perform manual calculations, making the chart more efficient and user-friendly for understanding member trends.

Please check your PR against these items:

• I've read and followed the Contributor Guide
• I've explained my change
• I've written an automated test to prove my change works

We appreciate your contribution! 🙏

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Linear Issue: NY-863

 

[cursor]

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch cursor/NY-863-add-net-change-tooltip-0f30

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[peterzimon]

Tested it, looks good to me