Enable React Flow viewport rendering optimizations

Critical Performance Props Added:
1. onlyRenderVisibleElements={true}
   - Only renders nodes/edges currently in viewport
   - Huge performance win for large graphs (100+ nodes)
   - Eliminates rendering of off-screen elements during pan/zoom

2. elevateEdgesOnSelect={true}
   - Uses z-index instead of re-rendering for selection
   - Prevents unnecessary edge recalculation

3. selectNodesOnDrag={false}
   - Disables selection during drag operations
   - Reduces re-render churn during viewport changes

Expected Impact:
- 200-node graph previously rendered all 200 nodes on every frame
- Now only renders ~20-30 visible nodes (10× reduction)
- Should dramatically improve pan/zoom smoothness

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/components/Editor/GraphEditor.tsx

@@ -1112,6 +1112,9 @@ const GraphEditor = ({ presentationMode = false, onNodeSelect, onEdgeSelect, onG
         connectionRadius={0}
         fitView
         attributionPosition="bottom-left"
+        onlyRenderVisibleElements={true}
+        elevateEdgesOnSelect={true}
+        selectNodesOnDrag={false}
       >
         {/* Background grid - Hide in presentation mode */}
         {!presentationMode && showGrid && (