feat: add minimal main.json as creator starter template

public/scenes/main.json

@@ -0,0 +1,25 @@
+{
+  "assetBase": "",
+  "startScene": "scene_1",
+  "variables": {},
+  "scenes": {
+    "scene_1": {
+      "id": "scene_1",
+      "videoUrl": "scene_1/video.mp4",
+      "choices": [
+        { "text": "选项 A", "targetScene": "scene_2" },
+        { "text": "选项 B", "targetScene": "ending" }
+      ]
+    },
+    "scene_2": {
+      "id": "scene_2",
+      "videoUrl": "scene_2/video.mp4",
+      "nextScene": "ending"
+    },
+    "ending": {
+      "id": "ending",
+      "videoUrl": "ending/video.mp4",
+      "choices": []
+    }
+  }
+}