feat: integrate AI panel, context menu, and ghost nodes in tree editor

- Add AI Assist panel toggle button to tree editor toolbar
- Wire EditorAIPanel alongside TreeEditorLayout with single-panel rule
- Thread onNodeContextMenu through TreeEditorLayout → FlowCanvas → FlowCanvasNode
- Add right-click context menu with Generate Branch, Explain Node, Delete actions
- Add ghost node detection (_suggestion flag) with dashed border + opacity styling
- Add Accept/Dismiss overlay buttons on ghost nodes for future suggestion handling

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

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

@@ -31,9 +31,10 @@ interface FlowCanvasProps {
   selectedNodeId: string | null
   onNodeSelect: (nodeId: string | null) => void
   onSelectAnswerType: (nodeId: string, type: 'decision' | 'action' | 'solution') => void
+  onNodeContextMenu?: (e: React.MouseEvent, nodeId: string) => void
 }
 
-function FlowCanvasInner({ selectedNodeId, onNodeSelect, onSelectAnswerType }: FlowCanvasProps) {
+function FlowCanvasInner({ selectedNodeId, onNodeSelect, onSelectAnswerType, onNodeContextMenu }: FlowCanvasProps) {
   const { fitView, setCenter } = useReactFlow()
   const { nodes: layoutNodes, edges: layoutEdges, collapsedNodeIds, toggleCollapse, onNodesMeasured } = useTreeLayout()
   const [minimapVisible, setMinimapVisible] = useState(true)
@@ -46,7 +47,7 @@ function FlowCanvasInner({ selectedNodeId, onNodeSelect, onSelectAnswerType }: F
         return {
           ...n,
           selected: n.id === selectedNodeId,
-          data: { ...data, onToggleCollapse: toggleCollapse },
+          data: { ...data, onToggleCollapse: toggleCollapse, onContextMenu: onNodeContextMenu },
         }
       }
       if (n.type === 'answerStub') {
@@ -59,7 +60,7 @@ function FlowCanvasInner({ selectedNodeId, onNodeSelect, onSelectAnswerType }: F
       }
       return n
     })
-  }, [layoutNodes, selectedNodeId, toggleCollapse, onSelectAnswerType])
+  }, [layoutNodes, selectedNodeId, toggleCollapse, onSelectAnswerType, onNodeContextMenu])
 
   const [nodes, setNodes, onNodesChange] = useNodesState(nodesWithCallbacks)
   const [edges, setEdges, onEdgesChange] = useEdgesState(layoutEdges)