feat(online): implement IchniOnline API integration with BestHTTP

- Add BestHTTP reference to IchniOnline.asmdef
- Create ApiClient.cs: HTTP singleton with JWT auto-injection
- Create ApiResponse.cs: ResponseCode enum, GlobalResponse<T>, ApiResult<T>
- Create AuthDtos.cs: ThirdPartyLoginRequestDto, LoginRequestDto, RegisterRequestDto, LoginResponseDto, UserResponseDto
- Create AuthService.cs: TapTap/password/register/logout flows with events
- Extend LoginCacheData.cs: JWT + server user data fields
- Extend LoginCacheManager.cs: SaveAuthSession, ClearSession, HasValidSession
- Extend ThirdPartyServiceManager.cs: OnLoginWithToken event for AuthService
- Update LoginPage.cs: Use AuthService with loading states and null-safe callbacks
- Update StartUIPage.cs: Use HasValidSession for session check

Fixes post-review:
- LoginPage: Add null check for Register success (response may be null)
- AuthService: Add try/catch around TapTap login API call

Assets/Scripts/Online/Network/Models/ApiResponse.cs

@@ -0,0 +1,82 @@
+namespace IchniOnline.Online.Network.Models
+{
+    using System;
+
+    [System.Serializable]
+    public enum ResponseCode
+    {
+        Ok = 10000,
+        BadRequest = 10400,
+        Unauthorized = 10401,
+        Forbidden = 10403,
+        NotFound = 10404,
+        InternalServerError = 10500
+    }
+
+    /// <summary>
+    /// Non-generic base class for Unity JsonUtility deserialization.
+    /// Concrete generic GlobalResponse<T> inherits from this.
+    /// </summary>
+    [System.Serializable]
+    public abstract class GlobalResponseBase
+    {
+        public ResponseCode Code;
+        public string Message;
+    }
+
+    /// <summary>
+    /// Generic server response wrapper. JsonUtility can deserialize this to the base class,
+    /// then cast to the concrete type for Data access.
+    /// </summary>
+    /// <typeparam name="T">Data payload type</typeparam>
+    [System.Serializable]
+    public class GlobalResponse<T> : GlobalResponseBase
+    {
+        public T Data;
+    }
+
+    /// <summary>
+    /// Unified API result wrapper with factory methods.
+    /// Note: JsonUtility doesn't support generic deserialization directly,
+    /// so use GlobalResponseBase for deserialization then wrap in ApiResult.
+    /// </summary>
+    /// <typeparam name="T">Data payload type</typeparam>
+    [System.Serializable]
+    public class ApiResult<T>
+    {
+        public bool IsSuccess => Code == ResponseCode.Ok;
+        public T Data { get; private set; }
+        public ResponseCode Code { get; private set; }
+        public string Message { get; private set; }
+        public string ErrorDetail { get; private set; }
+
+        private ApiResult() { }
+
+        public static ApiResult<T> Ok(T data)
+        {
+            return new ApiResult<T>
+            {
+                Data = data,
+                Code = ResponseCode.Ok,
+                Message = "Success",
+                ErrorDetail = null
+            };
+        }
+
+        public static ApiResult<T> Fail(ResponseCode code, string message, string detail = null)
+        {
+            return new ApiResult<T>
+            {
+                Data = default(T),
+                Code = code,
+                Message = message,
+                ErrorDetail = detail
+            };
+        }
+
+        public static ApiResult<T> Fail(int code, string message, string detail = null)
+        {
+            return Fail((ResponseCode)code, message, detail);
+        }
+    }
+}

Assets/Scripts/Online/Network/Models/ApiResponse.cs.meta

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+fileFormatVersion: 2
+guid: 476434455b6d65a4494c003a2ae54754

Assets/Scripts/Online/Network/Models/AuthDtos.cs

@@ -0,0 +1,46 @@
+using System;
+
+namespace IchniOnline.Online.Network.Models
+{
+    [Serializable]
+    public class ThirdPartyLoginRequestDto
+    {
+        public string Token;
+        public string TokenType;
+        public string MacKey;
+        public string MacAlgorithm;
+    }
+
+    [Serializable]
+    public class LoginRequestDto
+    {
+        public string Username;
+        public string EncryptedPassword;
+        public string SessionKey;
+    }
+
+    [Serializable]
+    public class RegisterRequestDto
+    {
+        public string Username;
+        public string Password;
+        public string DisplayName;
+    }
+
+    [Serializable]
+    public class LoginResponseDto
+    {
+        public string Token;
+        public UserResponseDto User;
+    }
+
+    [Serializable]
+    public class UserResponseDto
+    {
+        public string UserId;
+        public string Username;
+        public string DisplayName;
+        public string AvatarUrl;
+        public int Permission;
+    }
+}

Assets/Scripts/Online/Network/Models/AuthDtos.cs.meta

@@ -0,0 +1,2 @@
+fileFormatVersion: 2
+guid: 3d4424c69a64b3a43a48a432b2626ac9