Place brain neurons on whole pixels

The force simulation settles on arbitrary floats, and a neuron on a half pixel
has its ring, its gap and its disc rounded one edge at a time — which on the
smallest ones is a visibly off-centre ring. The size is already whole, so the
position was the last fractional term.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

frontend/src/components/Flow/BrainView.tsx

@@ -124,9 +124,14 @@ function build(graph: BrainGraph): { nodes: Node[]; edges: Edge[] } {
         id: node.id,
         type: "brain",
         // React Flow places by the top-left corner; the layout means centres.
+        // Rounded, because the force simulation settles on arbitrary floats and
+        // a neuron on a half pixel has its ring, its gap and its disc rounded
+        // one edge at a time — which on the smallest ones is a visibly
+        // off-centre ring. `size` is already whole, so this is the last
+        // fractional term.
         position: {
-          x: (node.x ?? 0) - node.size / 2,
-          y: (node.y ?? 0) - node.size / 2,
+          x: Math.round((node.x ?? 0) - node.size / 2),
+          y: Math.round((node.y ?? 0) - node.size / 2),
         },
         data: {
           label: source.label,