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/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