Adjust Stats plan quantity display in Billing History and Receipts

[bobmatyas]

This PR changes the display of Jetpack Stats plans on the Billing History and Receipts page to use "views" instead of "items". For example, instead of describing a plan as "100000 items" it displays it as "100,000 views".

Reported on Linear here:
https://linear.app/a8c/issue/JETH-9313/stats-stats-upgrades-confusing-on-purchase-page

Before:

After:

Proposed Changes

• Adjust the display of Stats plans by changing "items" to "views" and adding proper formatting.

Why are these changes being made?

• To fix an inconsistency in how the numbers are displayed in the Billing History and Receipts.

Testing Instructions

Make sure your WordPress.com account has a paid Jetpack Stats plan:

• Navigate to /me/purchases/billing (billing history page)
• Verify Jetpack Stats shows "Purchase for 250,000 views per month" (or the number from the plan you added)
• Click on a Jetpack Stats receipt at /me/purchases/billing/{id}
• Verify the line item shows the correct format with formatted numbers (commas)

Pre-merge Checklist

• [ x ] Has the general commit checklist been followed? (PCYsg-hS-p2)
• Have you written new tests for your changes?
• Have you tested the feature in Simple (P9HQHe-k8-p2), Atomic (P9HQHe-jW-p2), and self-hosted Jetpack sites (PCYsg-g6b-p2)?
• Have you checked for TypeScript, React or other console errors?
• Have you tested accessibility for your changes? Ensure the feature remains usable with various user agents (e.g., browsers), interfaces (e.g., keyboard navigation), and assistive technologies (e.g., screen readers) (PCYsg-S3g-p2).
• Have you used memoizing on expensive computations? More info in Memoizing with create-selector and Using memoizing selectors and Our Approach to Data
• Have we added the "[Status] String Freeze" label as soon as any new strings were ready for translation (p4TIVU-5Jq-p2)?
  • For UI changes, have we tested the change in various languages (for example, ES, PT, FR, or DE)? The length of text and words vary significantly between languages.
• For changes affecting Jetpack: Have we added the "[Status] Needs Privacy Updates" label if this pull request changes what data or activity we track or use (p4TIVU-aUh-p2)?

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