Increase ELK node spacing and revert frontend node height to 40

NODE_SPACING 40→60 gives edges more vertical room between nodes.
FIXED_H reverted to 40 to match backend NODE_HEIGHT.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/src/pages/routes/diagram/DiagramNode.tsx

@@ -3,7 +3,7 @@ import { getNodeStyle, isCompoundType } from './nodeStyles';
 import styles from './diagram.module.css';
 
 const FIXED_W = 200;
-const FIXED_H = 52;
+const FIXED_H = 40;
 const MAX_LABEL = 22;
 
 function truncateLabel(label: string | undefined): string {