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

components/LiveMiniPlayer.tsx

@@ -0,0 +1,185 @@
+import React, { useRef } from 'react';
+import {
+  View,
+  Text,
+  Image,
+  StyleSheet,
+  TouchableOpacity,
+  Animated,
+  PanResponder,
+  Dimensions,
+  Platform,
+} from 'react-native';
+import { useRouter } from 'expo-router';
+import { useSafeAreaInsets } from 'react-native-safe-area-context';
+import { Ionicons } from '@expo/vector-icons';
+import { useLiveStore } from '../store/liveStore';
+import { useVideoStore } from '../store/videoStore';
+import { proxyImageUrl } from '../utils/imageUrl';
+
+const MINI_W = 160;
+const MINI_H = 90;
+
+const LIVE_HEADERS = {
+  Referer: 'https://live.bilibili.com',
+  'User-Agent':
+    'Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/120.0.0.0 Safari/537.36',
+};
+
+export function LiveMiniPlayer() {
+  const { isActive, roomId, title, cover, hlsUrl, clearLive } = useLiveStore();
+  const videoMiniActive = useVideoStore(s => s.isActive);
+  const router = useRouter();
+  const insets = useSafeAreaInsets();
+  const pan = useRef(new Animated.ValueXY()).current;
+
+  const panResponder = useRef(
+    PanResponder.create({
+      onStartShouldSetPanResponder: () => true,
+      onPanResponderGrant: () => {
+        pan.setOffset({ x: (pan.x as any)._value, y: (pan.y as any)._value });
+        pan.setValue({ x: 0, y: 0 });
+      },
+      onPanResponderMove: Animated.event(
+        [null, { dx: pan.x, dy: pan.y }],
+        { useNativeDriver: false },
+      ),
+      onPanResponderRelease: () => {
+        pan.flattenOffset();
+        const { width: sw, height: sh } = Dimensions.get('window');
+        const curX = (pan.x as any)._value;
+        const curY = (pan.y as any)._value;
+        const clampedX = Math.max(-sw + MINI_W + 12, Math.min(12, curX));
+        const clampedY = Math.max(-sh + MINI_H + 60, Math.min(60, curY));
+        if (curX !== clampedX || curY !== clampedY) {
+          Animated.spring(pan, {
+            toValue: { x: clampedX, y: clampedY },
+            useNativeDriver: false,
+          }).start();
+        }
+      },
+    }),
+  ).current;
+
+  if (!isActive) return null;
+
+  // 视频 MiniPlayer 激活时，直播小窗叠放其上方（避免重叠）
+  const bottomOffset = insets.bottom + 16 + (videoMiniActive ? 106 : 0);
+
+  const handlePress = () => {
+    router.push(`/live/${roomId}` as any);
+  };
+
+  // Web 端降级：展示封面图 + LIVE 徽标
+  if (Platform.OS === 'web') {
+    return (
+      <Animated.View
+        style={[styles.container, { bottom: bottomOffset, transform: pan.getTranslateTransform() }]}
+        {...panResponder.panHandlers}
+      >
+        <TouchableOpacity style={styles.main} onPress={handlePress} activeOpacity={0.85}>
+          <Image source={{ uri: proxyImageUrl(cover) }} style={styles.videoArea} />
+          <View style={styles.liveBadge}>
+            <View style={styles.liveDot} />
+            <Text style={styles.liveText}>LIVE</Text>
+          </View>
+          <Text style={styles.titleText} numberOfLines={1}>{title}</Text>
+        </TouchableOpacity>
+        <TouchableOpacity style={styles.closeBtn} onPress={clearLive}>
+          <Ionicons name="close" size={14} color="#fff" />
+        </TouchableOpacity>
+      </Animated.View>
+    );
+  }
+
+  // Native：实际 HLS 流播放
+  const Video = require('react-native-video').default;
+
+  return (
+    <Animated.View
+      style={[styles.container, { bottom: bottomOffset, transform: pan.getTranslateTransform() }]}
+      {...panResponder.panHandlers}
+    >
+      <TouchableOpacity style={styles.main} onPress={handlePress} activeOpacity={0.85}>
+        <View style={styles.videoArea}>
+          <Video
+            key={hlsUrl}
+            source={{ uri: hlsUrl, headers: LIVE_HEADERS }}
+            style={StyleSheet.absoluteFill}
+            resizeMode="cover"
+            controls={false}
+            muted={false}
+            paused={false}
+            repeat={false}
+            onError={clearLive}
+          />
+        </View>
+        <View style={styles.liveBadge}>
+          <View style={styles.liveDot} />
+          <Text style={styles.liveText}>LIVE</Text>
+        </View>
+        <Text style={styles.titleText} numberOfLines={1}>{title}</Text>
+      </TouchableOpacity>
+      <TouchableOpacity style={styles.closeBtn} onPress={clearLive}>
+        <Ionicons name="close" size={14} color="#fff" />
+      </TouchableOpacity>
+    </Animated.View>
+  );
+}
+
+const styles = StyleSheet.create({
+  container: {
+    position: 'absolute',
+    right: 12,
+    width: MINI_W,
+    height: MINI_H,
+    borderRadius: 8,
+    backgroundColor: '#1a1a1a',
+    overflow: 'hidden',
+    elevation: 8,
+    shadowColor: '#000',
+    shadowOffset: { width: 0, height: 2 },
+    shadowOpacity: 0.3,
+    shadowRadius: 4,
+  },
+  main: { flex: 1 },
+  videoArea: {
+    width: '100%',
+    height: 66,
+    backgroundColor: '#111',
+  },
+  liveBadge: {
+    position: 'absolute',
+    top: 4,
+    left: 6,
+    flexDirection: 'row',
+    alignItems: 'center',
+    backgroundColor: 'rgba(0,0,0,0.55)',
+    paddingHorizontal: 5,
+    paddingVertical: 2,
+    borderRadius: 3,
+    gap: 3,
+  },
+  liveDot: { width: 5, height: 5, borderRadius: 2.5, backgroundColor: '#f00' },
+  liveText: { color: '#fff', fontSize: 9, fontWeight: '700', letterSpacing: 0.5 },
+  titleText: {
+    color: '#fff',
+    fontSize: 11,
+    paddingHorizontal: 6,
+    paddingVertical: 3,
+    lineHeight: 14,
+    height: 24,
+    backgroundColor: '#1a1a1a',
+  },
+  closeBtn: {
+    position: 'absolute',
+    top: 4,
+    right: 4,
+    width: 18,
+    height: 18,
+    borderRadius: 9,
+    backgroundColor: 'rgba(0,0,0,0.6)',
+    alignItems: 'center',
+    justifyContent: 'center',
+  },
+});