Grow the flow editor in from the centre, hide the empty brain band

The editor's mount-time fitView animated from React Flow's default viewport,
which read as the graph swiping in from the corner on every open. The first
fit is instant now, later ones stay animated, and a scaleIn wrapper gives the
same entrance the brain view has — with a re-measure on completion so the
handle bounds are not stored mid-scale.

Home only renders the brain band once some flow has nodes, so a fresh install
no longer reserves a screenful of empty space above the flows card.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HTsT1isxUjw5gtkJk8WhuA

frontend/src/routes/_layout/index.tsx

@@ -116,7 +116,10 @@ function Dashboard() {
     // Every section here is free to shrink instead. See DESIGN-GUIDELINES.md
     // → Responsive.
     <div className="grid gap-6 [&>*]:min-w-0">
-      <BrainView />
+      {/* Nothing wired up yet means nothing to draw, and the band would still
+          hold a screenful of empty space above the flows card. Node count
+          rather than flow count: a flow made a minute ago has none. */}
+      {flows.some((flow) => (flow.node_count ?? 0) > 0) ? <BrainView /> : null}
 
       <Card className="gap-0 py-0">
         {isPending ? (