simplify: replace manual indexes array with Array.from

[jordankrueger]

Summary

• Removes 2 lines from donationArray.readState() in playground/index.html
• Replaces a manual for loop building an indexes array with Array.from({ length: count }, (_, i) => ...) directly in the buttons property
• No behavior change — output is identical

Test plan

• Open playground locally, switch to Donation array tab
• Verify 3-button and 4-button configs render correctly
• Verify "Other" button appends correctly
• Amounts match button labels ($25, $50, $100, etc.)