feat: add S2A API client for dashboard, account, group, and proxy management with backend proxy handling.

frontend/src/api/s2a.ts

@@ -42,7 +42,19 @@ export class S2AClient {
       )
     }
 
-    return response.json()
+    const json = await response.json()
+
+    // S2A API 返回格式: { code: 0, message: "success", data: {...} }
+    // 需要提取 data 字段
+    if (json && typeof json === 'object' && 'code' in json && 'data' in json) {
+      if (json.code !== 0) {
+        throw new Error(json.message || 'API error')
+      }
+      return json.data as T
+    }
+
+    // 如果不是标准格式，直接返回
+    return json as T
   }
 
   // Dashboard APIs