feat: 双击播放队列中的歌曲（不清空队列）

- 新增 POST /api/player/:botId/play-at 端点，跳转到队列指定索引
- 使用 queue.playAt(index) + resolveAndPlay，保留队列完整
- Queue.vue 添加 @dblclick 处理，双击即播放对应歌曲
- 添加 store.playAtIndex() action

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

web/src/stores/player.ts

@@ -172,6 +172,13 @@ export const usePlayerStore = defineStore('player', {
       setTimeout(() => this.syncElapsed(), 500);
     },
 
+    async playAtIndex(index: number) {
+      if (!this.activeBotId) return;
+      await axios.post(`/api/player/${this.activeBotId}/play-at`, { index });
+      this.serverElapsed = 0;
+      this._syncAfterAction();
+    },
+
     async play(query: string, platform = 'netease') {
       if (!this.activeBotId) return;
       await axios.post(`/api/player/${this.activeBotId}/play`, { query, platform });