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/main.ts

@@ -0,0 +1,7 @@
+import { createApp } from 'vue'
+import { createPinia } from 'pinia'
+import EditorApp from './App.vue'
+
+const app = createApp(EditorApp)
+app.use(createPinia())
+app.mount('#editor-app')