The fastest way to get help is email: sakshamvirmani30@gmail.com. Include your iOS version and a description of what happened — screenshots help a lot.
Spark stores your data on-device and syncs through your private iCloud account. Check that you're signed into iCloud in Settings and that iCloud Drive is enabled for Spark under Settings → your name → iCloud. Sync can take a moment after reopening the app on another device.
If PRO isn't showing after a reinstall or on a new device, open Spark's settings and tap Restore Purchases while signed into the same Apple ID you bought with.
Subscriptions are billed by Apple. Manage or cancel any time in Settings → your name → Subscriptions. Refunds are handled by Apple at reportaproblem.apple.com.
Ideas and bug reports are genuinely welcome — email them over. The best ones tend to ship.