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

net_others.go

@@ -2,11 +2,14 @@
 
 package ctrld
 
-import "tailscale.com/net/netmon"
+import (
+	"context"
 
-// validInterfaces returns a set containing only default route interfaces.
-// TODO: deuplicated with cmd/cli/net_others.go in v2.
-func validInterfaces() map[string]struct{} {
+	"tailscale.com/net/netmon"
+)
+
+// ValidInterfaces returns a set containing only default route interfaces.
+func ValidInterfaces(_ context.Context) map[string]struct{} {
 	defaultRoute, err := netmon.DefaultRoute()
 	if err != nil {
 		return nil