feat: i18n for choice prompt via promptKey, with fallback

public/scenes/demo.json

@@ -211,6 +211,7 @@
           "text": "与陌生人握手",
           "textKey": "left_door.choice.handshake",
           "prompt": "陌生人会记住你的善意",
+          "promptKey": "left_door.prompt.handshake",
           "targetScene": "trust_ending",
           "effects": [
             { "type": "add", "target": "trust", "value": 30 }
@@ -322,6 +323,7 @@
           "text": "开启信任的旅程（需要 trust >= 80）",
           "textKey": "trust_ending.choice.journey",
           "prompt": "你们的羁绊将改变一切",
+          "promptKey": "trust_ending.prompt.journey",
           "targetScene": "secret_ending",
           "conditions": [
             { "variable": "trust", "op": ">=", "value": 80 }