feat(Auth): Unify shared PAM sync across greeter & lockscreen

- Add a neutral `dms auth sync` command and reuse the shared auth flow from:
- Settings auth toggle auto-apply
- `dms greeter sync`
- `dms greeter install`
- greeter auth cleanup paths

- Rework lockscreen PAM so DMS builds /etc/pam.d/dankshell from the system login stack, but removes fingerprint and U2F from that password path. Keep /etc/pam.d/dankshell-u2f separate.

- Preserve custom PAM files in place to avoid adding duplicate greeter auth when the distro already provides it, and keep NixOS on the non-writing path.

core/cmd/dms/commands_auth.go

@@ -0,0 +1,76 @@
+package main
+
+import (
+	"fmt"
+	"strings"
+
+	"github.com/AvengeMedia/DankMaterialShell/core/internal/log"
+	sharedpam "github.com/AvengeMedia/DankMaterialShell/core/internal/pam"
+	"github.com/spf13/cobra"
+)
+
+var authCmd = &cobra.Command{
+	Use:   "auth",
+	Short: "Manage DMS authentication sync",
+	Long:  "Manage shared PAM/authentication setup for DMS greeter and lock screen",
+}
+
+var authSyncCmd = &cobra.Command{
+	Use:   "sync",
+	Short: "Sync DMS authentication configuration",
+	Long:  "Apply shared PAM/authentication changes for the lock screen and greeter based on current DMS settings",
+	Run: func(cmd *cobra.Command, args []string) {
+		yes, _ := cmd.Flags().GetBool("yes")
+		term, _ := cmd.Flags().GetBool("terminal")
+		if term {
+			if err := syncAuthInTerminal(yes); err != nil {
+				log.Fatalf("Error launching auth sync in terminal: %v", err)
+			}
+			return
+		}
+		if err := syncAuth(yes); err != nil {
+			log.Fatalf("Error syncing authentication: %v", err)
+		}
+	},
+}
+
+func init() {
+	authSyncCmd.Flags().BoolP("yes", "y", false, "Non-interactive mode: skip prompts")
+	authSyncCmd.Flags().BoolP("terminal", "t", false, "Run auth sync in a new terminal (for entering sudo password)")
+}
+
+func syncAuth(nonInteractive bool) error {
+	if !nonInteractive {
+		fmt.Println("=== DMS Authentication Sync ===")
+		fmt.Println()
+	}
+
+	logFunc := func(msg string) {
+		fmt.Println(msg)
+	}
+
+	if err := sharedpam.SyncAuthConfig(logFunc, "", sharedpam.SyncAuthOptions{}); err != nil {
+		return err
+	}
+
+	if !nonInteractive {
+		fmt.Println("\n=== Authentication Sync Complete ===")
+		fmt.Println("\nAuthentication changes have been applied.")
+	}
+
+	return nil
+}
+
+func syncAuthInTerminal(nonInteractive bool) error {
+	syncFlags := make([]string, 0, 1)
+	if nonInteractive {
+		syncFlags = append(syncFlags, "--yes")
+	}
+
+	shellSyncCmd := "dms auth sync"
+	if len(syncFlags) > 0 {
+		shellSyncCmd += " " + strings.Join(syncFlags, " ")
+	}
+	shellCmd := shellSyncCmd + `; echo; echo "Authentication sync finished. Closing in 3 seconds..."; sleep 3`
+	return runCommandInTerminal(shellCmd)
+}