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

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

frontend/src/pages/NetworkDiagrams/index.tsx

@@ -260,6 +260,21 @@ export default function NetworkDiagramsPage() {
               {d.description && (
                 <p className="mb-3 line-clamp-2 text-xs text-muted-foreground">{d.description}</p>
               )}
+              {/* Thumbnail preview */}
+              {d.thumbnail_url ? (
+                <div className="mb-2 overflow-hidden rounded border border-default">
+                  <img
+                    src={d.thumbnail_url}
+                    alt={d.name}
+                    className="h-[120px] w-full object-cover"
+                    onError={e => { (e.target as HTMLImageElement).style.display = 'none' }}
+                  />
+                </div>
+              ) : (
+                <div className="mb-2 flex h-[120px] items-center justify-center rounded border border-default bg-elevated">
+                  <Network size={32} className="text-muted-foreground/30" />
+                </div>
+              )}
               {d.node_count > 0 && (
                 <div className="mb-2">
                   <TopologyBar categoryCounts={d.category_counts} nodeCount={d.node_count} />