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

package.json

@@ -9,6 +9,9 @@
     "preview": "vite preview"
   },
   "dependencies": {
+    "@vue-flow/background": "^1.3.2",
+    "@vue-flow/controls": "^1.1.3",
+    "@vue-flow/core": "^1.48.2",
     "dexie": "^4.4.3",
     "pinia": "^2.1.0",
     "vue": "^3.4.0"