Replace most occurrences of os.Exit(0) in the backend and sync packages with a custom backend.Exit function that can allow "soft exit" in GUI/TUI environments to avoid killing the entire process; currently all major errors still result in killing the entire process

src/backend/copy.go

@@ -64,7 +64,7 @@ func ClipClearArgument() {
 		oldContents := clipScanner.Text()
 		clipClear(oldContents)
 	} else {
-		os.Exit(0)
+		os.Exit(0) // use os.Exit instead of backend.Exit, as this function runs out of a background subprocess that is invisible to the user (will never appear in GUI/TUI environment)
 	}
 }