Display native macOS notifications for payment failures

[robcoward]

Summary

errand-cloud is integrating Stripe for paid subscriptions. When a renewal payment fails, the errand-server forwards a subscription_alert event to errand-desktop. The app needs to display native macOS notifications so the user is immediately aware of payment issues.

Changes Required

• Handle subscription_alert events received from errand-server
• Display native macOS notifications with content varying by alert type:
  • payment_failed (retrying): "Payment failed for {plan} subscription. Retrying {date}."
  • payment_failed (final): "Payment failed for {plan} subscription. Your subscription has expired."
  • payment_succeeded: "Payment successful. Your {plan} subscription has been renewed."

Impact

• New case in the event dispatch for subscription_alert type
• Uses existing macOS notification infrastructure (UNUserNotificationCenter)
• No new dependencies expected

OpenSpec Change

payment-failure-notifications - 0/11 tasks, unstarted