fix(client): 修复图片上传 content-type 被覆盖问题

BaseOptions 不应写死 Content-Type: application/json，
Dio 会根据 data 类型自动选择（FormData → multipart，Map → json）

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

client/lib/core/api/api_client.dart

@@ -10,7 +10,6 @@ final _publicDio = Dio(BaseOptions(
   baseUrl: AppConfig.apiBaseUrl,
   connectTimeout: const Duration(seconds: 5),
   receiveTimeout: const Duration(seconds: 15),
-  headers: {'Content-Type': 'application/json'},
 ));
 
 final apiClientProvider = Provider<ApiClient>((ref) {
@@ -55,7 +54,6 @@ class ApiClient {
       connectTimeout: const Duration(seconds: 5),
       receiveTimeout: const Duration(seconds: 15),
       headers: {
-        'Content-Type': 'application/json',
         if (token != null) 'Authorization': 'Bearer $token',
       },
     ));