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/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;
+    }
+}