feat: 性能优化 + Bug修复 + 搜索增强

- expo-image 替换 RN Image（VideoCard/LiveCard/BigVideoCard/CommentItem，recyclingKey）
- DanmakuList Animated.Value 对象池，减少 GC 压力
- FlatList 性能参数：windowSize=7 / maxToRenderPerBatch=6 / removeClippedSubviews
- bilibili.ts 请求去重（getVideoDetail/getPlayUrl）
- SESSDATA 迁移至 expo-secure-store（utils/secureStorage.ts，启动自动迁移）
- 主题系统扩展：新增 sheetBg/modalBg/modalText/placeholder/iconDefault/danger 等 token
- 多组件深色模式适配：DownloadSheet/LivePlayer/NativeVideoPlayer/DownloadProgressBtn/CommentItem/LoginModal
- 搜索页增强：搜索建议 + 热搜榜（hooks/useSearch.ts + app/search.tsx）
- LoginModal 修复轮询竞态（cancelled flag + try-catch）
- 修复 downloads.tsx 冗余三元表达式

components/DownloadProgressBtn.tsx

@@ -2,6 +2,7 @@ import React from 'react';
 import { View, TouchableOpacity, StyleSheet } from 'react-native';
 import { Ionicons } from '@expo/vector-icons';
 import { useDownloadStore } from '../store/downloadStore';
+import { useTheme } from '../utils/theme';
 
 const SIZE = 32;   // 环外径
 const RING = 3;    // 环宽
@@ -13,6 +14,7 @@ interface Props {
 }
 
 export function DownloadProgressBtn({ onPress }: Props) {
+  const theme = useTheme();
   const tasks = useDownloadStore((s) => s.tasks);
   const downloading = Object.values(tasks).filter((t) => t.status === 'downloading');
   const hasDownloading = downloading.length > 0;
@@ -32,7 +34,7 @@ export function DownloadProgressBtn({ onPress }: Props) {
       <Ionicons
         name="cloud-download-outline"
         size={22}
-        color={hasDownloading ? BLUE : '#999'}
+        color={hasDownloading ? BLUE : theme.iconDefault}
       />
     </TouchableOpacity>
   );