chore: editor App and SceneGraph updates

editor/App.vue

@@ -122,6 +122,7 @@ onMounted(() => loadDemo())
           @select-node="store.selectedNodeId = $event"
           @add-edge="editor.onAddEdge"
           @test-scene="testScene"
+          @clear-selection="store.selectedNodeId = null"
         />
         <div v-else-if="loading" class="loading-hint">加载剧情数据…</div>
         <PreviewPanel v-if="showPreview" :video-url="previewVideoUrl" />

editor/components/SceneGraph.vue

@@ -20,11 +20,12 @@ const emit = defineEmits<{
   selectNode: [id: string]
   addEdge: [source: string, target: string]
   testScene: [id: string]
+  clearSelection: []
 }>()
 
 const nodes = ref<any[]>([])
 const edges = ref<any[]>([])
-const { onNodeClick, onConnect, onNodeContextMenu, onNodeDragStop, fitView } = useVueFlow()
+const { onNodeClick, onConnect, onNodeContextMenu, onNodeDragStop, onPaneClick, fitView } = useVueFlow()
 const ctxMenuVisible = ref(false)
 const ctxMenuX = ref(0)
 const ctxMenuY = ref(0)
@@ -135,6 +136,8 @@ onNodeDragStop((ev) => {
   savePosition(ev.node.id, Math.round(pos.x), Math.round(pos.y))
 })
 
+onPaneClick(() => emit('clearSelection'))
+
 onConnect((conn: Connection) => {
   if (conn.source && conn.target) emit('addEdge', conn.source, conn.target)
 })