feat: 深色模式完善 + UP主主页 + 缓存管理

- 深色模式：补全 settings 页选项按钮、退出登录按钮主题色
- UP主主页：新增 /creator/[mid] 路由，展示 UP 主信息、粉丝数、视频列表
  - bilibili.ts 新增 getUploaderInfo / getUploaderVideos API
  - 视频详情页 UP 主行可点击跳转主页
- 缓存管理：settings 页新增「存储」分区，显示缓存大小并支持一键清除
  - 新增 utils/cache.ts（计算大小 + 清除 expo-image/文件系统缓存）

utils/secureStorage.ts

@@ -0,0 +1,32 @@
+import AsyncStorage from '@react-native-async-storage/async-storage';
+import { Platform } from 'react-native';
+
+// expo-secure-store is only available on native platforms
+let SecureStore: typeof import('expo-secure-store') | null = null;
+if (Platform.OS !== 'web') {
+  try { SecureStore = require('expo-secure-store'); } catch {}
+}
+
+/** Read a sensitive value from SecureStore (native) or AsyncStorage (web fallback). */
+export async function getSecure(key: string): Promise<string | null> {
+  if (SecureStore) {
+    return SecureStore.getItemAsync(key);
+  }
+  return AsyncStorage.getItem(key);
+}
+
+export async function setSecure(key: string, value: string): Promise<void> {
+  if (SecureStore) {
+    await SecureStore.setItemAsync(key, value);
+  } else {
+    await AsyncStorage.setItem(key, value);
+  }
+}
+
+export async function deleteSecure(key: string): Promise<void> {
+  if (SecureStore) {
+    await SecureStore.deleteItemAsync(key);
+  } else {
+    await AsyncStorage.removeItem(key);
+  }
+}