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

cmd/cli/net_others.go

@@ -3,10 +3,7 @@
 package cli
 
 import (
-	"context"
 	"net"
-
-	"tailscale.com/net/netmon"
 )
 
 // patchNetIfaceName patches network interface names on non-Linux/Darwin platforms
@@ -14,12 +11,3 @@ func patchNetIfaceName(iface *net.Interface) (bool, error) { return true, nil }
 
 // validInterface checks if an interface is valid on non-Linux/Darwin platforms
 func validInterface(iface *net.Interface, validIfacesMap map[string]struct{}) bool { return true }
-
-// validInterfacesMap returns a set containing only default route interfaces.
-func validInterfacesMap(ctx context.Context) map[string]struct{} {
-	defaultRoute, err := netmon.DefaultRoute()
-	if err != nil {
-		return nil
-	}
-	return map[string]struct{}{defaultRoute.InterfaceName: {}}
-}