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

store/authStore.ts

@@ -1,6 +1,7 @@
 import { create } from 'zustand';
 import AsyncStorage from '@react-native-async-storage/async-storage';
 import { getUserInfo } from '../services/bilibili';
+import { getSecure, setSecure, deleteSecure } from '../utils/secureStorage';
 
 interface AuthState {
   sessdata: string | null;
@@ -22,21 +23,33 @@ export const useAuthStore = create<AuthState>((set) => ({
   isLoggedIn: false,
 
   login: async (sessdata, uid, username) => {
+    await setSecure('SESSDATA', sessdata);
     await AsyncStorage.multiSet([
-      ['SESSDATA', sessdata],
       ['UID', uid],
       ['USERNAME', username ?? ''],
     ]);
+    // Migrate: remove SESSDATA from AsyncStorage if it was there before
+    await AsyncStorage.removeItem('SESSDATA').catch(() => {});
     set({ sessdata, uid, username: username ?? null, isLoggedIn: true });
   },
 
   logout: async () => {
-    await AsyncStorage.multiRemove(['SESSDATA', 'UID', 'USERNAME', 'FACE']);
+    await deleteSecure('SESSDATA');
+    await AsyncStorage.multiRemove(['UID', 'USERNAME', 'FACE']);
     set({ sessdata: null, uid: null, username: null, face: null, isLoggedIn: false });
   },
 
   restore: async () => {
-    const sessdata = await AsyncStorage.getItem('SESSDATA');
+    // Try SecureStore first, fallback to AsyncStorage for migration
+    let sessdata = await getSecure('SESSDATA');
+    if (!sessdata) {
+      sessdata = await AsyncStorage.getItem('SESSDATA');
+      if (sessdata) {
+        // Migrate from AsyncStorage to SecureStore
+        await setSecure('SESSDATA', sessdata);
+        await AsyncStorage.removeItem('SESSDATA');
+      }
+    }
     if (sessdata) {
       set({ sessdata, isLoggedIn: true });
       try {