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/App.vue

@@ -12,13 +12,15 @@ const videoElB = ref<HTMLVideoElement | null>(null)
 const loading = ref(true)
 const started = ref(false)
 const showMenu = ref(false)
+const hasAutoSave = ref(false)
 
-const { loadGame, start, makeChoice, saveGame, loadGameFromSlot, refreshSaves } =
+const { loadGame, start, resumeAutoSave, makeChoice, saveGame, loadGameFromSlot, refreshSaves, saveSystem } =
   useGameEngine(() => [videoElA.value, videoElB.value])
 
 async function init() {
   await loadGame('/scenes/demo.json')
   loading.value = false
+  hasAutoSave.value = (await saveSystem.load(0)) !== null
 }
 
 function handleStart() {
@@ -26,6 +28,11 @@ function handleStart() {
   start()
 }
 
+async function handleResume() {
+  started.value = true
+  await resumeAutoSave()
+}
+
 function onVideoReady(elA: HTMLVideoElement, elB: HTMLVideoElement) {
   videoElA.value = elA
   videoElB.value = elB
@@ -72,6 +79,7 @@ init()
       </div>
       <div v-if="!started" class="start-overlay">
         <button class="start-btn" @click="handleStart">开始游戏</button>
+        <button v-if="hasAutoSave" class="start-btn resume-btn" @click="handleResume">继续上次进度</button>
       </div>
       <div v-if="store.gameEnded" class="game-end-overlay">
         <div class="game-end-text">游戏结束</div>
@@ -190,4 +198,10 @@ html, body {
 .start-btn:hover {
   background: rgba(255, 255, 255, 0.25);
 }
+
+.resume-btn {
+  margin-top: 16px;
+  border-color: rgba(100, 200, 255, 0.3);
+  color: #8cf;
+}
 </style>