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_linux.go

@@ -1,15 +1,7 @@
 package cli
 
 import (
-	"context"
 	"net"
-	"net/netip"
-	"os"
-	"strings"
-
-	"tailscale.com/net/netmon"
-
-	"github.com/Control-D-Inc/ctrld"
 )
 
 // patchNetIfaceName patches network interface names on Linux
@@ -23,45 +15,3 @@ func validInterface(iface *net.Interface, validIfacesMap map[string]struct{}) bo
 	_, ok := validIfacesMap[iface.Name]
 	return ok
 }
-
-// validInterfacesMap returns a set containing non virtual interfaces.
-// This filters out virtual interfaces to ensure DNS is only configured on physical interfaces
-func validInterfacesMap(ctx context.Context) map[string]struct{} {
-	m := make(map[string]struct{})
-	vis := virtualInterfaces(ctx)
-	netmon.ForeachInterface(func(i netmon.Interface, prefixes []netip.Prefix) {
-		if _, existed := vis[i.Name]; existed {
-			return
-		}
-		m[i.Name] = struct{}{}
-	})
-	// Fallback to the default route interface if found nothing.
-	// This ensures we always have at least one interface to configure
-	if len(m) == 0 {
-		defaultRoute, err := netmon.DefaultRoute()
-		if err != nil {
-			return m
-		}
-		m[defaultRoute.InterfaceName] = struct{}{}
-	}
-	return m
-}
-
-// virtualInterfaces returns a map of virtual interfaces on the current machine.
-// This reads from /sys/devices/virtual/net to identify virtual network interfaces
-// Virtual interfaces should not have DNS configured as they don't represent physical network connections
-func virtualInterfaces(ctx context.Context) map[string]struct{} {
-	logger := ctrld.LoggerFromCtx(ctx)
-	s := make(map[string]struct{})
-	entries, err := os.ReadDir("/sys/devices/virtual/net")
-	if err != nil {
-		logger.Error().Err(err).Msg("Failed to read /sys/devices/virtual/net")
-		return nil
-	}
-	for _, entry := range entries {
-		if entry.IsDir() {
-			s[strings.TrimSpace(entry.Name())] = struct{}{}
-		}
-	}
-	return s
-}