feat: auto-save on scene change + resume from auto-save

- useGameEngine: auto-save to slot 0 on every sceneChange event
- useGameEngine: add resumeAutoSave() for continuing from auto-save
- useGameEngine: extract registerEvents() to share between start and resume
- SaveLoadMenu: show slot 0 as '自动存档' with distinct styling and read-only button
- App.vue: check auto-save on load, show '继续上次进度' button if available

src/composables/useGameEngine.ts

@@ -20,14 +20,12 @@ export function useGameEngine(videoEls: () => [HTMLVideoElement | null, HTMLVide
     engine.stateManager.init(data.variables)
   }
 
-  function start() {
-    const [elA, elB] = videoEls()
-    engine.videoManager.attach(elA!, elB!)
-
+  function registerEvents() {
     engine.on('sceneChange', (scene) => {
       store.setScene(scene)
       store.clearChoices()
       store.clearTimer()
+      saveGame(0)
     })
 
     engine.on('choiceRequest', (choiceList) => {
@@ -49,10 +47,22 @@ export function useGameEngine(videoEls: () => [HTMLVideoElement | null, HTMLVide
       store.setGameEnded(true)
       engine.choiceSystem.stop()
     })
+  }
 
+  function start() {
+    const [elA, elB] = videoEls()
+    engine.videoManager.attach(elA!, elB!)
+    registerEvents()
     engine.start()
   }
 
+  async function resumeAutoSave(): Promise<boolean> {
+    const [elA, elB] = videoEls()
+    engine.videoManager.attach(elA!, elB!)
+    registerEvents()
+    return await loadGameFromSlot(0)
+  }
+
   function makeChoice(index: number) {
     const scene = store.currentScene
     if (!scene?.choices) return
@@ -99,5 +109,5 @@ export function useGameEngine(videoEls: () => [HTMLVideoElement | null, HTMLVide
     destroy()
   })
 
-  return { loadGame, start, makeChoice, saveGame, loadGameFromSlot, refreshSaves, engine, saveSystem }
+  return { loadGame, start, resumeAutoSave, makeChoice, saveGame, loadGameFromSlot, refreshSaves, engine, saveSystem }
 }