Fix blank portal: add root element heights, use dagre for graph layout

- Set explicit height on html/body/#root so React Flow container renders
- Replace @dagrejs/dagre (broken CJS dynamic require) with dagre v0.8.5
- Hierarchical top-down layout with dagre, arrow markers on edges
- Move React Flow controls to top-right to avoid floating button overlap

portal/src/App.tsx

@@ -44,7 +44,9 @@ export default function App() {
   return (
     <div className="h-screen w-screen overflow-hidden relative">
       {/* Full-screen graph */}
-      <GraphView selectedId={selectedId} onSelect={selectNode} />
+      <div className="absolute inset-0">
+        <GraphView selectedId={selectedId} onSelect={selectNode} />
+      </div>
 
       {/* Floating action buttons — bottom-left */}
       <div className="fixed bottom-5 left-5 flex flex-col gap-2 z-30">