Add networking layer and wire up v9 login flow with unit tests.

Introduce APIClient/AuthAPI, complete login with multi-account selection, and polish the login card UI with shadow and keyboard dismiss.

Co-authored-by: Cursor <cursoragent@cursor.com>

suixinkan/Core/Networking/APIEnvelope.swift

@@ -0,0 +1,60 @@
+//
+//  APIEnvelope.swift
+//  suixinkan
+//
+
+import Foundation
+
+/// 后端统一响应包裹实体，承载业务 code、msg 和真实 data。
+struct APIEnvelope<T: Decodable>: Decodable {
+    let data: T?
+    let code: Int
+    let msg: String?
+
+    /// 判断后端业务 code 是否代表成功。
+    var isSuccess: Bool {
+        code == 100000
+    }
+
+    /// 后端统一 Envelope 的 JSON 字段映射。
+    enum CodingKeys: String, CodingKey {
+        case data
+        case code
+        case msg
+    }
+
+    /// 自定义解码逻辑，兼容成功响应、空 data 和 EmptyPayload。
+    init(from decoder: Decoder) throws {
+        let container = try decoder.container(keyedBy: CodingKeys.self)
+        code = try container.decodeIfPresent(Int.self, forKey: .code) ?? 0
+        msg = try container.decodeIfPresent(String.self, forKey: .msg)
+
+        guard code == 100000 else {
+            data = nil
+            return
+        }
+
+        guard container.contains(.data), try !container.decodeNil(forKey: .data) else {
+            data = nil
+            return
+        }
+
+        if T.self == EmptyPayload.self {
+            data = EmptyPayload() as? T
+            return
+        }
+
+        data = try container.decode(T.self, forKey: .data)
+    }
+}
+
+/// 空响应实体，用于表示接口成功但不返回业务 data。
+struct EmptyPayload: Codable {}
+
+/// 错误响应实体，用于从 HTTP 错误体中提取服务端文案。
+struct ErrorEnvelope: Decodable {
+    let code: Int?
+    let msg: String?
+    let message: String?
+    let error: String?
+}