iOS suspends the process on backgrounding, freezing the core's network
threads so in-flight transfers stall and never fire notifications. Hold a
UIApplication background-task assertion (BackgroundActivityController) while
transfers/shares are active so iOS grants its grace window — long enough to
finish and notify. Released on foreground, on completion, or on expiration.
No UIBackgroundModes added (keeps App Store validation clean); macOS is a
no-op since it already runs unfocused.
Add a localized iOS-only Settings notice explaining the platform limit so it
doesn't read as a bug.
