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/APIRequest.swift

@@ -0,0 +1,53 @@
+//
+//  APIRequest.swift
+//  suixinkan
+//
+
+import Foundation
+
+/// HTTP 方法实体，限制网络层支持的请求方法。
+enum HTTPMethod: String {
+    case get = "GET"
+    case post = "POST"
+    case put = "PUT"
+    case delete = "DELETE"
+}
+
+/// 强类型 API 请求实体，描述一次请求的方法、路径、参数、Header 和响应类型。
+nonisolated struct APIRequest<Response: Decodable> {
+    var method: HTTPMethod
+    var path: String
+    var queryItems: [URLQueryItem]
+    var headers: [String: String]
+    var body: AnyEncodable?
+
+    /// 创建一个 API 请求，并把可编码请求体擦除为统一的 AnyEncodable。
+    init<Body: Encodable>(
+        method: HTTPMethod,
+        path: String,
+        queryItems: [URLQueryItem] = [],
+        headers: [String: String] = [:],
+        body: Body? = Optional<EmptyPayload>.none
+    ) {
+        self.method = method
+        self.path = path
+        self.queryItems = queryItems
+        self.headers = headers
+        self.body = body.map(AnyEncodable.init)
+    }
+}
+
+/// Encodable 类型擦除包装器，用于在 APIRequest 中保存任意请求体。
+nonisolated struct AnyEncodable: Encodable, Sendable {
+    private let encodeValue: @Sendable (Encoder) throws -> Void
+
+    init<Value: Encodable>(_ value: Value) {
+        encodeValue = { encoder in
+            try value.encode(to: encoder)
+        }
+    }
+
+    func encode(to encoder: Encoder) throws {
+        try encodeValue(encoder)
+    }
+}