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/MockURLSession.swift

@@ -0,0 +1,63 @@
+//
+//  MockURLSession.swift
+//  suixinkanTests
+//
+
+import Foundation
+@testable import suixinkan
+
+/// 按顺序返回预设响应的 URLSession 测试替身。
+final class MockURLSession: URLSessionProtocol {
+    private(set) var requests: [URLRequest] = []
+    private var responses: [Result<(Data, URLResponse), Error>]
+    private var responseIndex = 0
+
+    init(responses: [Data], statusCode: Int = 200) {
+        self.responses = responses.map { data in
+            .success((
+                data,
+                HTTPURLResponse(
+                    url: URL(string: "https://api-test.zhifly.cn/mock")!,
+                    statusCode: statusCode,
+                    httpVersion: nil,
+                    headerFields: nil
+                )!
+            ))
+        }
+    }
+
+    init(results: [Result<(Data, URLResponse), Error>]) {
+        self.responses = results
+    }
+
+    func data(for request: URLRequest) async throws -> (Data, URLResponse) {
+        requests.append(request)
+        guard responseIndex < responses.count else {
+            throw URLError(.badServerResponse)
+        }
+        defer { responseIndex += 1 }
+        return try responses[responseIndex].get()
+    }
+}
+
+enum TestJSON {
+    static func envelope<T: Encodable>(data: T) throws -> Data {
+        let payload = try JSONEncoder().encode(data)
+        let payloadObject = try JSONSerialization.jsonObject(with: payload)
+        let envelope: [String: Any] = [
+            "code": 100000,
+            "msg": "success",
+            "data": payloadObject,
+        ]
+        return try JSONSerialization.data(withJSONObject: envelope)
+    }
+
+    static func errorEnvelope(code: Int, msg: String) throws -> Data {
+        let envelope: [String: Any] = [
+            "code": code,
+            "msg": msg,
+            "data": NSNull(),
+        ]
+        return try JSONSerialization.data(withJSONObject: envelope)
+    }
+}