fix: add start button to satisfy browser autoplay policy

Replace auto-start after loading with a '开始游戏' button overlay.
The user click provides the required user gesture for autoplay with sound,
fixing the issue where play() was silently rejected and choices never appeared.

src/App.vue

@@ -1,5 +1,5 @@
 <script setup lang="ts">
-import { ref, nextTick } from 'vue'
+import { ref } from 'vue'
 import GamePlayer from '@/components/GamePlayer.vue'
 import ChoicePanel from '@/components/ChoicePanel.vue'
 import { useGameEngine } from '@/composables/useGameEngine'
@@ -8,13 +8,17 @@ import { useGameStore } from '@/stores/gameStore'
 const store = useGameStore()
 const videoElRef = ref<HTMLVideoElement | null>(null)
 const loading = ref(true)
+const started = ref(false)
 
 const { loadGame, start, makeChoice } = useGameEngine(() => videoElRef.value)
 
 async function init() {
   await loadGame('/scenes/demo.json')
   loading.value = false
-  await nextTick()
+}
+
+function handleStart() {
+  started.value = true
   start()
 }
 
@@ -37,6 +41,9 @@ init()
         <GamePlayer @video-ready="onVideoReady" />
         <ChoicePanel :choices="store.choices" @choose="onChoose" />
       </div>
+      <div v-if="!started" class="start-overlay">
+        <button class="start-btn" @click="handleStart">开始游戏</button>
+      </div>
       <div v-if="store.gameEnded" class="game-end-overlay">
         <div class="game-end-text">游戏结束</div>
       </div>
@@ -101,4 +108,30 @@ html, body {
   font-size: 36px;
   letter-spacing: 4px;
 }
+
+.start-overlay {
+  position: fixed;
+  inset: 0;
+  background: rgba(0, 0, 0, 0.85);
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  z-index: 100;
+}
+
+.start-btn {
+  padding: 18px 48px;
+  font-size: 20px;
+  color: #fff;
+  background: rgba(255, 255, 255, 0.12);
+  border: 1px solid rgba(255, 255, 255, 0.3);
+  border-radius: 4px;
+  cursor: pointer;
+  letter-spacing: 4px;
+  transition: background 0.2s;
+}
+
+.start-btn:hover {
+  background: rgba(255, 255, 255, 0.25);
+}
 </style>