fix(design): preserve existing canvas nodes when compressed snapshot omits them

SaveCanvas treated a nil Nodes/Connections slice from a compressed snapshot
as an empty canvas, wiping existing content. Distinguish nil (field omitted,
keep the project's current nodes/connections) from an explicit empty slice.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

server/internal/application/asset_cleanup_test.go

@@ -182,6 +182,60 @@ func TestSaveCanvasAcceptsCompressedSnapshotPayload(t *testing.T) {
 	}
 }
 
+func TestSaveCanvasPreservesNodesWhenCompressedSnapshotOmitsThem(t *testing.T) {
+	ctx := context.Background()
+	repo := memory.NewProjectRepository()
+	service := NewDesignService(repo, nil, nil, nil, nil)
+	service.now = func() time.Time {
+		return time.Unix(101, 0).UTC()
+	}
+	project := design.Project{
+		ID:         "project-null-snapshot-nodes",
+		Title:      "Null snapshot nodes",
+		Status:     design.StatusReady,
+		UpdatedAt:  time.Unix(100, 0).UTC(),
+		Version:    "100000",
+		SnapshotID: "snapshot-old",
+		Viewport:   design.Viewport{X: 1, Y: 2, K: 1},
+		Nodes: []design.Node{
+			{ID: "existing-image", Type: design.NodeTypeImage, Title: "Existing", Content: "http://localhost:19000/canvas-assets/existing.png", Status: "success"},
+		},
+		Connections: []design.Connection{
+			{ID: "existing-connection", FromNodeID: "existing-image", ToNodeID: "existing-image"},
+		},
+	}
+	if err := repo.Save(ctx, project); err != nil {
+		t.Fatal(err)
+	}
+	snapshotProject := project
+	snapshotProject.SnapshotID = "snapshot-client"
+	snapshotProject.Viewport = design.Viewport{X: 12, Y: 34, K: 0.8}
+	snapshotProject.Nodes = nil
+	snapshotProject.Connections = nil
+	canvas, err := design.EncodeCanvasSnapshot(snapshotProject)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	saved, err := service.SaveCanvas(ctx, project.ID, design.Viewport{}, nil, nil, "", "", canvas)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if saved.SnapshotID != "snapshot-client" {
+		t.Fatalf("expected client snapshot id, got %s", saved.SnapshotID)
+	}
+	if saved.Viewport != snapshotProject.Viewport {
+		t.Fatalf("unexpected viewport: %#v", saved.Viewport)
+	}
+	if len(saved.Nodes) != 1 || saved.Nodes[0].ID != "existing-image" {
+		t.Fatalf("expected existing nodes to be preserved, got %#v", saved.Nodes)
+	}
+	if len(saved.Connections) != 1 || saved.Connections[0].ID != "existing-connection" {
+		t.Fatalf("expected existing connections to be preserved, got %#v", saved.Connections)
+	}
+}
+
 func TestSaveCanvasDoesNotDeleteImageAssetReferencedByAgentMessage(t *testing.T) {
 	ctx := context.Background()
 	repo := memory.NewProjectRepository()

server/internal/application/design_service.go

@@ -1082,10 +1082,18 @@ func (s *DesignService) SaveCanvas(ctx context.Context, projectID string, viewpo
 			return design.Project{}, fmt.Errorf("%w: invalid canvas snapshot", design.ErrInvalidInput)
 		}
 		if len(nodes) == 0 {
-			nodes = snapshot.Nodes
+			if snapshot.Nodes != nil {
+				nodes = snapshot.Nodes
+			} else {
+				nodes = append([]design.Node(nil), project.Nodes...)
+			}
 		}
 		if len(connections) == 0 {
-			connections = snapshot.Connections
+			if snapshot.Connections != nil {
+				connections = snapshot.Connections
+			} else {
+				connections = append([]design.Connection(nil), project.Connections...)
+			}
 		}
 		if viewport.K == 0 {
 			viewport = snapshot.Viewport