fix: Flex with multiple slices opened was always showing slice A s-meter.

singleinstance_other.go

@@ -0,0 +1,7 @@
+//go:build !windows
+
+package main
+
+// acquireSingleInstance is a no-op off Windows (the single-instance guard uses a
+// Windows named mutex). Always allows the app to start.
+func acquireSingleInstance() bool { return true }