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>

suixinkanTests/APIErrorTests.swift

@@ -0,0 +1,29 @@
+//
+//  APIErrorTests.swift
+//  suixinkanTests
+//
+
+import XCTest
+@testable import suixinkan
+
+/// 网络错误测试，覆盖登录凭证失效判断。
+final class APIErrorTests: XCTestCase {
+    func testAuthenticationExpiredForUnauthorizedHTTPStatus() {
+        XCTAssertTrue(APIError.isAuthenticationExpired(APIError.httpStatus(401, "Unauthorized")))
+        XCTAssertTrue(APIError.isAuthenticationExpired(APIError.httpStatus(403, "Forbidden")))
+    }
+
+    func testAuthenticationExpiredForAndroidTokenInvalidCodes() {
+        for code in [180024, 100091, 100090, 100060] {
+            XCTAssertTrue(
+                APIError.isAuthenticationExpired(APIError.serverCode(code, "token 失效")),
+                "code \(code) 应识别为 token 失效"
+            )
+        }
+    }
+
+    func testNetworkErrorIsNotAuthenticationExpired() {
+        XCTAssertFalse(APIError.isAuthenticationExpired(APIError.networkFailed("请求超时，请稍后重试")))
+        XCTAssertFalse(APIError.isAuthenticationExpired(APIError.serverCode(100001, "密码错误")))
+    }
+}