feat: add glow edge system with directional selection animation

Custom bezier edges with gradient glow for the flow editor:
- Default: subtle white/gray gradient with soft glow
- Downstream (cyan): animated flowing dashes from selected node through subtree
- Upstream (amber): animated flow from selected node back to root
- Cross-reference: dashed cyan with arrow markers
- SVG gradient + filter defs for performant rendering

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

frontend/src/components/tree-editor/FlowCanvas.tsx

@@ -12,10 +12,10 @@ import {
   PanOnScrollMode,
   type NodeMouseHandler,
 } from '@xyflow/react'
-import '@xyflow/react/dist/style.css'
 
 import { FlowCanvasNode, NODE_TYPE_CONFIG } from './FlowCanvasNode'
 import { FlowCanvasAnswerNode } from './FlowCanvasAnswerNode'
+import { GlowEdge, GlowEdgeDefs } from './GlowEdge'
 import { useTreeLayout } from './useTreeLayout'
 import { cn } from '@/lib/utils'
 import { Map as MapIcon, MapPinOff } from 'lucide-react'
@@ -27,6 +27,10 @@ const nodeTypes = {
   answerStub: FlowCanvasAnswerNode,
 }
 
+const edgeTypes = {
+  glowEdge: GlowEdge,
+}
+
 interface FlowCanvasProps {
   selectedNodeId: string | null
   onNodeSelect: (nodeId: string | null) => void
@@ -36,7 +40,7 @@ interface FlowCanvasProps {
 
 function FlowCanvasInner({ selectedNodeId, onNodeSelect, onSelectAnswerType, onNodeContextMenu }: FlowCanvasProps) {
   const { fitView, setCenter } = useReactFlow()
-  const { nodes: layoutNodes, edges: layoutEdges, collapsedNodeIds, toggleCollapse, onNodesMeasured } = useTreeLayout()
+  const { nodes: layoutNodes, edges: layoutEdges, collapsedNodeIds, toggleCollapse, onNodesMeasured } = useTreeLayout(selectedNodeId)
   const [minimapVisible, setMinimapVisible] = useState(true)
 
   // Inject callbacks into node data (because useTreeLayout creates placeholder functions)
@@ -124,6 +128,7 @@ function FlowCanvasInner({ selectedNodeId, onNodeSelect, onSelectAnswerType, onN
         onNodesChange={onNodesChange}
         onEdgesChange={onEdgesChange}
         nodeTypes={nodeTypes}
+        edgeTypes={edgeTypes}
         onNodeClick={handleNodeClick}
         onPaneClick={handlePaneClick}
         fitView
@@ -139,6 +144,7 @@ function FlowCanvasInner({ selectedNodeId, onNodeSelect, onSelectAnswerType, onN
         proOptions={{ hideAttribution: true }}
         className="dark bg-accent/30"
       >
+        <GlowEdgeDefs />
         <Background variant={BackgroundVariant.Dots} gap={20} size={1.5} color="oklch(0.63 0.02 260 / 0.25)" />
         <Controls showInteractive={false} className="bg-card! border-border! shadow-lg!" />
         {minimapVisible && (