feat(network): thumbnail generation on save, shown on list page

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/pages/NetworkDiagrams/DiagramEditor.tsx

@@ -334,21 +334,51 @@ function DiagramEditorInner() {
         nodes: serializeNodes(),
         edges: serializeEdges(),
       }
+      let savedId: string | null = diagramIdRef.current
       if (diagramIdRef.current) {
         await networkDiagramsApi.update(diagramIdRef.current, payload)
       } else {
         const created = await networkDiagramsApi.create(payload)
+        savedId = created.id
         setDiagramId(created.id)
         navigate(`/network-diagrams/${created.id}`, { replace: true })
       }
       setIsDirty(false)
       setLastSavedAt(new Date())
+
+      // Generate thumbnail in the background — don't block save UX on failure
+      if (savedId && nodes.length > 0) {
+        try {
+          const { toPng } = await import('html-to-image')
+          const THUMB_W = 480
+          const THUMB_H = 300
+          const bounds = getNodesBounds(nodes)
+          const viewport = getViewportForBounds(bounds, THUMB_W, THUMB_H, 0.5, 2, 0.1)
+          const flowEl = document.querySelector('.react-flow__viewport') as HTMLElement | null
+          if (flowEl) {
+            const dataUrl = await toPng(flowEl, {
+              backgroundColor: '#16181f',
+              width: THUMB_W,
+              height: THUMB_H,
+              style: {
+                width: `${THUMB_W}px`,
+                height: `${THUMB_H}px`,
+                transform: `translate(${viewport.x}px, ${viewport.y}px) scale(${viewport.zoom})`,
+                transformOrigin: 'top left',
+              },
+            })
+            await networkDiagramsApi.uploadThumbnail(savedId, dataUrl)
+          }
+        } catch {
+          // Thumbnail failure is silent — doesn't affect save success
+        }
+      }
     } catch {
       toast.error('Failed to save diagram')
     } finally {
       setIsSaving(false)
     }
-  }, [name, clientName, assetName, description, serializeNodes, serializeEdges, navigate])
+  }, [name, clientName, assetName, description, serializeNodes, serializeEdges, navigate, nodes])
 
   useEffect(() => {
     const interval = setInterval(() => {