refactor: consolidate network interface detection logic

Move platform-specific network interface detection from cmd/cli/ to root package
as ValidInterfaces function. This eliminates code duplication and provides a
consistent interface for determining valid physical network interfaces across
all platforms.

- Remove duplicate validInterfacesMap functions from platform-specific files
- Add context parameter to virtualInterfaces for proper logging
- Update all callers to use ctrld.ValidInterfaces instead of local functions
- Improve error handling in virtual interface detection on Linux

nameservers_windows.go

@@ -444,7 +444,3 @@ func ValidInterfaces(ctx context.Context) map[string]struct{} {
 	}
 	return m
 }
-
-func validInterfaces() map[string]struct{} {
-	return ValidInterfaces(context.Background())
-}