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/App/NetworkServices.swift

@@ -0,0 +1,32 @@
+//
+//  NetworkServices.swift
+//  suixinkan
+//
+
+import Foundation
+
+@MainActor
+/// 全局网络依赖容器，集中提供 APIClient 与 AuthAPI。
+final class NetworkServices {
+    static let shared = NetworkServices()
+
+    let apiClient: APIClient
+    let authAPI: AuthAPI
+
+    /// 生产环境默认初始化。
+    private init() {
+        let client = APIClient(environment: .current)
+        apiClient = client
+        authAPI = AuthAPI(client: client)
+        client.bindAuthTokenProvider {
+            let token = AppStore.shared.token.trimmingCharacters(in: .whitespacesAndNewlines)
+            return token.isEmpty ? nil : token
+        }
+    }
+
+    /// 测试专用初始化，允许注入 mock `APIClient`。
+    init(apiClient: APIClient) {
+        self.apiClient = apiClient
+        authAPI = AuthAPI(client: apiClient)
+    }
+}