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/components/SaveLoadMenu.vue

@@ -21,6 +21,23 @@ const maxSlots = 5
       <h2 class="save-title">存档 / 读档</h2>
 
       <div class="slot-list">
+        <div class="save-slot auto-save-slot">
+          <div class="slot-label auto-save-label">自动存档</div>
+          <div class="slot-info" v-if="saves.find(s => s.slot === 0)">
+            {{ saves.find(s => s.slot === 0)!.sceneLabel }}
+          </div>
+          <div class="slot-info empty" v-else>暂无自动存档</div>
+          <div class="slot-actions">
+            <button
+              class="slot-btn load-btn"
+              :disabled="!saves.find(s => s.slot === 0)"
+              @click="emit('load', 0)"
+            >
+              读取
+            </button>
+          </div>
+        </div>
+
         <div
           v-for="slot in maxSlots"
           :key="slot"
@@ -94,6 +111,15 @@ const maxSlots = 5
   border-radius: 4px;
 }
 
+.auto-save-slot {
+  border-color: rgba(100, 200, 255, 0.3);
+  background: rgba(100, 200, 255, 0.06);
+}
+
+.auto-save-label {
+  color: #6cf;
+}
+
 .slot-label {
   font-size: 14px;
   color: #aaa;