fix(mp): 存储层语义统一 + UTF-16 截断修复

- secureGet: 增加 TextEncoder/TextDecoder 替代 charCodeAt 避免 UTF-16 截断
- secureGet: _es_ 前缀键返回空时增加明文键 fallback（对齐 storageGet 语义）
- request.ts safeGet / auth.ts storageGet: 简化为直接委托 secureGet

apps/miniprogram/src/services/request.ts

@@ -21,11 +21,7 @@ const ERROR_CODE_MAP: Record<string, string> = {
 };
 
 function safeGet(key: string): string {
-  try {
-    return secureGet(key);
-  } catch {
-    return Taro.getStorageSync(key) || '';
-  }
+  return secureGet(key);
 }
 
 // --- Concurrency limiter ---