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/cache.ts

@@ -0,0 +1,63 @@
+import { Platform } from 'react-native';
+import * as FileSystem from 'expo-file-system/legacy';
+
+/** Get total size (bytes) of a directory recursively */
+async function dirSize(dirUri: string): Promise<number> {
+  try {
+    const info = await FileSystem.getInfoAsync(dirUri);
+    if (!info.exists) return 0;
+    const entries = await FileSystem.readDirectoryAsync(dirUri);
+    let total = 0;
+    for (const entry of entries) {
+      const entryUri = dirUri.endsWith('/') ? `${dirUri}${entry}` : `${dirUri}/${entry}`;
+      const entryInfo = await FileSystem.getInfoAsync(entryUri, { size: true });
+      if (!entryInfo.exists) continue;
+      if (entryInfo.isDirectory) {
+        total += await dirSize(entryUri);
+      } else {
+        total += (entryInfo as any).size ?? 0;
+      }
+    }
+    return total;
+  } catch {
+    return 0;
+  }
+}
+
+/** Format bytes to human-readable string */
+export function formatBytes(bytes: number): string {
+  if (bytes < 1024) return `${bytes} B`;
+  if (bytes < 1024 * 1024) return `${(bytes / 1024).toFixed(1)} KB`;
+  if (bytes < 1024 * 1024 * 1024) return `${(bytes / 1024 / 1024).toFixed(1)} MB`;
+  return `${(bytes / 1024 / 1024 / 1024).toFixed(2)} GB`;
+}
+
+/** Calculate image cache size (expo-image uses its own cache dirs) */
+export async function getImageCacheSize(): Promise<number> {
+  if (Platform.OS === 'web') return 0;
+  const cacheDir = FileSystem.cacheDirectory ?? '';
+  return dirSize(cacheDir);
+}
+
+/** Clear expo-image disk cache + expo-file-system cache directory */
+export async function clearImageCache(): Promise<void> {
+  if (Platform.OS === 'web') return;
+  try {
+    const { Image } = require('expo-image');
+    await Image.clearDiskCache();
+    await Image.clearMemoryCache();
+  } catch {}
+  // Also clear the general cache directory (MPD files, QR codes, etc.)
+  try {
+    const cacheDir = FileSystem.cacheDirectory ?? '';
+    const entries = await FileSystem.readDirectoryAsync(cacheDir);
+    await Promise.all(
+      entries.map(async entry => {
+        const uri = `${cacheDir}${entry}`;
+        try {
+          await FileSystem.deleteAsync(uri, { idempotent: true });
+        } catch {}
+      })
+    );
+  } catch {}
+}

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);
+  }
+}