feat: P15 ending gallery, chapter recap, visited tracking, save system v6

public/scenes/demo.json

@@ -31,6 +31,11 @@
       "condition": { "variable": "completed_game", "op": ">=", "value": 1 }
     }
   ],
+  "endings": [
+    { "id": "trust_end", "label": "信任的伙伴", "sceneId": "trust_ending", "thumbnail": "/images/end_trust.jpg" },
+    { "id": "alone_end", "label": "独行之路", "sceneId": "alone_ending", "thumbnail": "/images/end_alone.jpg" },
+    { "id": "continue_end", "label": "继续前行", "sceneId": "continue_ending", "thumbnail": "/images/end_continue.jpg" }
+  ],
   "chapters": [
     {
       "id": "ch1",