fix: normalize main flow section to (0,0) origin in frontend

The root cause of the Y-offset: ELK places main flow nodes at
arbitrary positions (e.g., y=679) within its root graph, and the
frontend rendered them at those raw positions. Handler sections were
already normalized via shiftNodes, but the main section was not.

Now useDiagramData.ts applies the same normalization to the main
section: computes bounding box, shifts nodes and edges so the section
starts at (0,0). This fixes the Y-offset regardless of what ELK
produces internally.

Removed the backend normalizePositions (was ineffective because handler
nodes at y=12 dominated the global minimum, preventing meaningful shift
of main flow nodes at y=679).

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

ui/src/components/ProcessDiagram/useDiagramData.ts

@@ -55,20 +55,30 @@ export function useDiagramData(
       e => mainNodeIds.has(e.sourceId) && mainNodeIds.has(e.targetId),
     );
 
-    // Compute main section bounding box
+    // Normalize main section to start at (0, 0) — ELK can place nodes
+    // at arbitrary positions within its root graph
     const mainBounds = computeBounds(mainNodes);
+    const mainOffX = mainBounds.minX;
+    const mainOffY = mainBounds.minY;
+    const shiftedMainNodes = shiftNodes(mainNodes, mainOffX, mainOffY);
+    const shiftedMainEdges = mainEdges.map(e => ({
+      ...e,
+      points: e.points.map(p => [p[0] - mainOffX, p[1] - mainOffY]),
+    }));
+    const mainWidth = mainBounds.maxX - mainBounds.minX;
+    const mainHeight = mainBounds.maxY - mainBounds.minY;
 
     const sections: DiagramSection[] = [
       {
         label: 'Main Route',
-        nodes: mainNodes,
-        edges: mainEdges,
+        nodes: shiftedMainNodes,
+        edges: shiftedMainEdges,
         offsetY: 0,
       },
     ];
 
-    let currentY = mainBounds.maxY + SECTION_GAP;
-    let maxWidth = mainBounds.maxX;
+    let currentY = mainHeight + SECTION_GAP;
+    let maxWidth = mainWidth;
 
     const addHandlerSections = (
       handlers: { label: string; nodes: DiagramNode[] }[],
@@ -106,7 +116,7 @@ export function useDiagramData(
     // Then error handlers
     addHandlerSections(errorSections, 'error');
 
-    const totalWidth = Math.max(layout.width ?? 0, mainBounds.maxX, maxWidth);
+    const totalWidth = Math.max(layout.width ?? 0, mainWidth, maxWidth);
     const totalHeight = currentY;
 
     return { sections, totalWidth, totalHeight };