fix(ios): APIClient 兼容有无 ResponseInterceptor 两种响应格式

服务器部署版本可能未启用 ResponseInterceptor，返回裸 JSON 而非
{ success, data, timestamp } 信封格式。APIClient 解码时先探测
JSON 结构，自动适配包裹/裸数据两种格式。

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

AIStudyApp/AIStudyApp/Core/Network/APIClient.swift

@@ -63,12 +63,7 @@ actor APIClient {
 
         switch httpResponse.statusCode {
         case 200, 201:
-            do {
+            return try decodeResponse(data)
-                let envelope = try JSONDecoder().decode(APIEnvelope<T>.self, from: data)
-                return envelope.data
-            } catch {
-                throw APIError.decodingFailed(error.localizedDescription)
-            }
         case 401 where !isRetry:
             if let newToken = await refreshAccessToken() {
                 self.token = newToken
@@ -109,6 +104,25 @@ actor APIClient {
             NotificationCenter.default.post(name: .tokenExpired, object: nil)
         }
     }
+
+    // MARK: - Decoding
+
+    private func decodeResponse<T: Decodable>(_ data: Data) throws -> T {
+        let decoder = JSONDecoder()
+        // Try unwrapped response first (no envelope), then wrapped
+        if let json = try? JSONSerialization.jsonObject(with: data) as? [String: Any],
+           json["data"] == nil && json["success"] == nil {
+            return try decoder.decode(T.self, from: data)
+        }
+        // Has envelope wrapper
+        do {
+            let envelope = try decoder.decode(APIEnvelope<T>.self, from: data)
+            return envelope.data
+        } catch {
+            // Fallback: try decoding T directly (e.g. server returns unwrapped on some endpoints)
+            return try decoder.decode(T.self, from: data)
+        }
+    }
 }
 
 // MARK: - Helper for encoding arbitrary Encodable