feat: P3 - visual scenario editor with Vue Flow

- editor/: stand-alone Vite multi-page app for visual scenario editing
- editor/components/SceneGraph.vue: Vue Flow graph with scene nodes, branch/default/QTE edges
- editor/components/NodeEditor.vue: right panel editing video/subtitle paths, choices, QTE params
- editor/components/PreviewPanel.vue: embedded video player previewing selected scene
- editor/composables/useGraphEditor.ts: bidirectional graph<->JSON sync
- editor/App.vue: toolbar (new scene, import/export JSON, load demo, start scene selector)
- @vue-flow/core|background|controls: graph visualization dependencies
- vite.config.ts: multi-page build (main + editor)
- ROADMAP: mark P3 as completed

editor/composables/useGraphEditor.ts

@@ -0,0 +1,121 @@
+import { ref, computed } from 'vue'
+import type { GameData, SceneNode, Choice } from '@engine/types'
+
+export interface EditorNode {
+  id: string
+  label: string
+  videoUrl: string
+  subtitleUrl: string
+  choices: Choice[]
+  nextScene: string
+  onEnter: any[]
+  qte: any | null
+}
+
+export function useGraphEditor() {
+  const gameData = ref<GameData>({ scenes: {}, startScene: '', variables: {} })
+  const selectedNodeId = ref<string | null>(null)
+  const startSceneId = ref('')
+
+  const selectedNode = computed(() => {
+    if (!selectedNodeId.value) return null
+    return gameData.value.scenes[selectedNodeId.value] ?? null
+  })
+
+  const sceneList = computed(() => {
+    return Object.values(gameData.value.scenes).map((s) => ({
+      id: s.id,
+      label: s.id,
+    }))
+  })
+
+  function loadJSON(json: GameData) {
+    gameData.value = JSON.parse(JSON.stringify(json))
+    startSceneId.value = json.startScene
+  }
+
+  function exportJSON(): GameData {
+    return JSON.parse(JSON.stringify({ ...gameData.value, startScene: startSceneId.value }))
+  }
+
+  function generateId(): string {
+    let i = Object.keys(gameData.value.scenes).length + 1
+    while (gameData.value.scenes[`scene_${i}`]) i++
+    return `scene_${i}`
+  }
+
+  function addScene(): string {
+    const id = generateId()
+    gameData.value.scenes[id] = {
+      id,
+      videoUrl: '',
+      choices: [],
+      nextScene: '',
+      subtitleUrl: '',
+      onEnter: [],
+    }
+    return id
+  }
+
+  function deleteScene(id: string) {
+    if (startSceneId.value === id) return
+    delete gameData.value.scenes[id]
+    for (const s of Object.values(gameData.value.scenes)) {
+      s.choices = (s.choices || []).filter((c) => c.targetScene !== id)
+      if (s.nextScene === id) s.nextScene = ''
+    }
+    if (selectedNodeId.value === id) selectedNodeId.value = null
+  }
+
+  function updateScene(id: string, partial: Partial<EditorNode>) {
+    const scene = gameData.value.scenes[id]
+    if (!scene) return
+    Object.assign(scene, partial)
+    gameData.value.scenes = { ...gameData.value.scenes }
+  }
+
+  function addChoice(sourceId: string) {
+    const scene = gameData.value.scenes[sourceId]
+    if (!scene) return
+    if (!scene.choices) scene.choices = []
+    scene.choices.push({
+      text: '新选项',
+      targetScene: '',
+    })
+    gameData.value.scenes = { ...gameData.value.scenes }
+  }
+
+  function updateChoice(sourceId: string, index: number, partial: Partial<Choice>) {
+    const scene = gameData.value.scenes[sourceId]
+    if (!scene?.choices) return
+    Object.assign(scene.choices[index], partial)
+    gameData.value.scenes = { ...gameData.value.scenes }
+  }
+
+  function deleteChoice(sourceId: string, index: number) {
+    const scene = gameData.value.scenes[sourceId]
+    if (!scene?.choices) return
+    scene.choices.splice(index, 1)
+    gameData.value.scenes = { ...gameData.value.scenes }
+  }
+
+  function newSceneData(): EditorNode {
+    return {
+      id: '',
+      label: '',
+      videoUrl: '',
+      subtitleUrl: '',
+      choices: [],
+      nextScene: '',
+      onEnter: [],
+      qte: null,
+    }
+  }
+
+  return {
+    gameData, selectedNodeId, selectedNode, sceneList, startSceneId,
+    loadJSON, exportJSON, addScene, deleteScene, updateScene,
+    addChoice, updateChoice, deleteChoice,
+    newSceneData, generateId,
+  }
+}