refactor(auth): relay on distro system auth over DMS managed sessions

- Gate greeter external-auth status fprint availability only on confirmed setups
- Reload dankshell-U2F/U2F-Key watchers live

Related: #2874
Port 1.5

core/internal/qmlchecks/greeter_auth_test.go

@@ -0,0 +1,25 @@
+package qmlchecks
+
+import (
+	"os"
+	"strings"
+	"testing"
+)
+
+func TestGreeterExternalAuthStatusUsesEffectiveFingerprintAvailability(t *testing.T) {
+	data, err := os.ReadFile("../../../quickshell/Modules/Greetd/GreeterContent.qml")
+	if err != nil {
+		t.Fatalf("read greeter QML: %v", err)
+	}
+
+	content := string(data)
+	for _, required := range []string{
+		"readonly property bool greeterPamHasExternalAuth: greeterPamHasFprint || greeterPamHasU2f",
+		"if (greeterPamHasFprint && greeterPamHasU2f)",
+		"if (greeterPamHasFprint)",
+	} {
+		if !strings.Contains(content, required) {
+			t.Fatalf("greeter external-auth status must contain %q", required)
+		}
+	}
+}