OpenSaaS for Wasp 0.19

Notable changes since Wasp 0.18.0:

• Added Polar as an new payment provider alongside Stripe and Lemon Squeezy. (#461)
• Completely overhauled file uploads with a two-step process (S3 URL generation → database
  storage), file deletion with S3 cleanup, and better error handling. (#472)
• Replaced react-hot-toast with a custom toast system built on @radix-ui/react-toast for better accessibility and consistent design across the application. (#472)

wasp-v0.18-template

Migrate Open Saas to 0.18.0 (#500)