[client] Classify session extend errors for the UI

Route services.Session errors through the same classifier Connection
uses so RequestExtend/WaitExtend return a structured ClientError with a
clean localized short message instead of the raw daemon error. Extract
the shared errorClassifier into errors.go, and fall back to the gRPC
status code when no message substring matches, since the daemon now
forwards a clean desc without the English marker text.

client/ui/services/errors_test.go

@@ -0,0 +1,50 @@
+//go:build !android && !ios && !freebsd && !js
+
+package services
+
+import (
+	"errors"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+	gcodes "google.golang.org/grpc/codes"
+	gstatus "google.golang.org/grpc/status"
+)
+
+func TestErrorClassifier_Classify(t *testing.T) {
+	c := errorClassifier{} // nil translator → Short is the bare "error.<code>" key
+
+	t.Run("permission denied by gRPC code with a clean desc", func(t *testing.T) {
+		// The daemon now forwards the innermost status: code + clean desc that
+		// no longer carries the English "permission denied" marker.
+		err := gstatus.Error(gcodes.PermissionDenied, "peer is already registered by a different User or a Setup Key")
+
+		ce := c.classify(err)
+		require.NotNil(t, ce)
+		require.Equal(t, "permission_denied", ce.Code)
+		require.Equal(t, "error.permission_denied", ce.Short)
+		require.Equal(t, "peer is already registered by a different User or a Setup Key", ce.Long)
+	})
+
+	t.Run("substring match still wins for unclassified codes", func(t *testing.T) {
+		err := gstatus.Error(gcodes.Unknown, "peer login has expired")
+
+		ce := c.classify(err)
+		require.NotNil(t, ce)
+		require.Equal(t, "session_expired", ce.Code)
+	})
+
+	t.Run("unavailable code maps to daemon_unreachable", func(t *testing.T) {
+		ce := c.classify(gstatus.Error(gcodes.Unavailable, "transport closing"))
+		require.Equal(t, "daemon_unreachable", ce.Code)
+	})
+
+	t.Run("unmatched stays unknown", func(t *testing.T) {
+		ce := c.classify(errors.New("something odd"))
+		require.Equal(t, "unknown", ce.Code)
+	})
+
+	t.Run("nil error", func(t *testing.T) {
+		require.Nil(t, c.classify(nil))
+	})
+}