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

vite.config.ts

@@ -10,4 +10,12 @@ export default defineConfig({
       '@engine': resolve(__dirname, 'engine'),
     },
   },
+  build: {
+    rollupOptions: {
+      input: {
+        main: resolve(__dirname, 'index.html'),
+        editor: resolve(__dirname, 'editor/index.html'),
+      },
+    },
+  },
 })