feat: [US-023] - Add choice node from toolbar

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/app/editor/[projectId]/FlowchartEditor.tsx

@@ -97,8 +97,21 @@ function FlowchartEditorInner({ initialData }: FlowchartEditorProps) {
   }, [getViewportCenter, setNodes])
 
   const handleAddChoice = useCallback(() => {
-    // TODO: Implement in US-023
+    const position = getViewportCenter()
-  }, [])
+    const newNode: Node = {
+      id: nanoid(),
+      type: 'choice',
+      position,
+      data: {
+        prompt: '',
+        options: [
+          { id: nanoid(), label: '' },
+          { id: nanoid(), label: '' },
+        ],
+      },
+    }
+    setNodes((nodes) => [...nodes, newNode])
+  }, [getViewportCenter, setNodes])
 
   const handleAddVariable = useCallback(() => {
     // TODO: Implement in US-026