feat: 直播小窗播放（PiP）功能

- 新增 store/liveStore.ts：Zustand store，存储小窗直播状态（roomId/title/cover/hlsUrl）
- 新增 components/LiveMiniPlayer.tsx：可拖动悬浮迷你直播播放器
  - 原生端实际播放 HLS 流（react-native-video，有声）
  - Web 端降级展示封面图 + LIVE 徽标
  - LIVE 红点徽标 + 标题条底部
  - 关闭按钮（×）和点击进入直播间
  - HLS 出错时自动关闭（onError → clearLive）
  - 视频 MiniPlayer 激活时自动上移避免重叠
- _layout.tsx：全局挂载 <LiveMiniPlayer />
- live/[roomId].tsx：顶部栏添加小窗按钮（browsers-outline）
  - 仅直播中且有流地址时显示，离线时占位保持 title 居中
  - 点击后 setLive + router.back()，返回首页继续看直播
  - 进入同房间时自动 clearLive() 避免双播

store/liveStore.ts

@@ -0,0 +1,23 @@
+import { create } from 'zustand';
+
+interface LiveStore {
+  isActive: boolean;
+  roomId: number;
+  title: string;
+  cover: string;       // room.keyframe（直播截图）用于 web 端降级封面
+  hlsUrl: string;
+  setLive: (roomId: number, title: string, cover: string, hlsUrl: string) => void;
+  clearLive: () => void;
+}
+
+export const useLiveStore = create<LiveStore>(set => ({
+  isActive: false,
+  roomId: 0,
+  title: '',
+  cover: '',
+  hlsUrl: '',
+  setLive: (roomId, title, cover, hlsUrl) =>
+    set({ isActive: true, roomId, title, cover, hlsUrl }),
+  clearLive: () =>
+    set({ isActive: false, roomId: 0, title: '', cover: '', hlsUrl: '' }),
+}));