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/Features/Auth/AuthSessionHelper.swift

@@ -0,0 +1,26 @@
+//
+//  AuthSessionHelper.swift
+//  suixinkan
+//
+
+import Foundation
+
+/// 登录会话辅助工具，负责写入本地 Token 并广播登录成功通知。
+enum AuthSessionHelper {
+
+    /// 完成登录，保存 Token 与用户偏好，并通知 SceneDelegate 切换根页面。
+    static func completeLogin(
+        with response: V9AuthResponse,
+        username: String,
+        privacyAgreementAccepted: Bool
+    ) {
+        let token = response.token.trimmingCharacters(in: .whitespacesAndNewlines)
+        guard !token.isEmpty else { return }
+
+        AppStore.shared.saveToken(token)
+        AppStore.shared.lastLoginUsername = username
+        AppStore.shared.privacyAgreementAccepted = privacyAgreementAccepted
+
+        NotificationCenter.default.post(name: NotificationName.userDidLogin, object: nil)
+    }
+}