Refactor admin login to cookie auth

src/TaxBaik.Web/Pages/Admin/Login.cshtml

@@ -64,33 +64,3 @@
     </div>
 </section>
 
-@if (Model.LoginSucceeded && Model.TokenPair is not null)
-{
-    <script>
-        (function () {
-            const tokenPair = @Html.Raw(System.Text.Json.JsonSerializer.Serialize(
-                Model.TokenPair,
-                new System.Text.Json.JsonSerializerOptions { PropertyNamingPolicy = System.Text.Json.JsonNamingPolicy.CamelCase }));
-            const expiryTicks = 621355968000000000 + ((Date.now() + (tokenPair.expiresIn || 3600) * 1000) * 10000);
-            localStorage.setItem('accessToken', tokenPair.accessToken || '');
-            localStorage.setItem('refreshToken', tokenPair.refreshToken || '');
-            localStorage.setItem('tokenExpiry', String(expiryTicks));
-            @if (Model.RememberMe)
-            {
-                <text>
-                localStorage.setItem('admin-remembered-username', @Html.Raw(System.Text.Json.JsonSerializer.Serialize(Model.Username)));
-                localStorage.setItem('admin-remember-checkbox', 'true');
-                </text>
-            }
-            else
-            {
-                <text>
-                localStorage.removeItem('admin-remembered-username');
-                localStorage.removeItem('admin-remember-checkbox');
-                </text>
-            }
-
-            window.location.replace(@Html.Raw(System.Text.Json.JsonSerializer.Serialize(Model.RedirectUrl)));
-        })();
-    </script>
-}

src/TaxBaik.Web/Pages/Admin/Login.cshtml.cs

@@ -1,7 +1,8 @@
 using System.ComponentModel.DataAnnotations;
+using System.Security.Claims;
 using Microsoft.AspNetCore.Mvc;
 using Microsoft.AspNetCore.Mvc.RazorPages;
-using TaxBaik.Web.Endpoints.Auth;
+using Microsoft.AspNetCore.Authentication;
 using TaxBaik.Web.Services;
 
 namespace TaxBaik.Web.Pages.Admin;
@@ -27,11 +28,6 @@ public class LoginModel(AuthService authService) : PageModel
     public string? ReturnUrl { get; set; }
 
     public string? ErrorMessage { get; set; }
-
-    public bool LoginSucceeded { get; set; }
-
-    public TokenPairResponse? TokenPair { get; set; }
-
     public string RedirectUrl { get; set; } = "/admin/dashboard";
 
     public void OnGet()
@@ -49,28 +45,32 @@ public class LoginModel(AuthService authService) : PageModel
             return Page();
         }
 
-        var authResult = await authService.AuthenticateAndGenerateTokenPairAsync(Username, Password);
-        if (!authResult.IsSuccess || authResult.TokenPair is null)
+        var user = await authService.AuthenticateAsync(Username, Password);
+        if (user is null)
         {
-            ErrorMessage = authResult.FailureReason switch
-            {
-                AuthFailureReason.DatabaseUnavailable => "로그인 서비스를 일시적으로 사용할 수 없습니다. 잠시 후 다시 시도하세요.",
-                _ => "아이디 또는 비밀번호가 올바르지 않습니다."
-            };
+            ErrorMessage = "아이디 또는 비밀번호가 올바르지 않습니다.";
             return Page();
         }
 
-        LoginSucceeded = true;
         RedirectUrl = NormalizeRedirectUrl(ReturnUrl);
-        TokenPair = new TokenPairResponse
+
+        var claims = new List<Claim>
         {
-            AccessToken = authResult.TokenPair.AccessToken,
-            RefreshToken = authResult.TokenPair.RefreshToken,
-            ExpiresIn = authResult.TokenPair.ExpiresIn,
-            Token = authResult.TokenPair.AccessToken
+            new(ClaimTypes.NameIdentifier, user.Id.ToString()),
+            new(ClaimTypes.Name, user.Username),
+            new(ClaimTypes.Role, "Admin")
         };
 
-        return Page();
+        var identity = new ClaimsIdentity(claims, AdminAuthDefaults.Scheme);
+        var principal = new ClaimsPrincipal(identity);
+        var properties = new AuthenticationProperties
+        {
+            IsPersistent = RememberMe,
+            RedirectUri = RedirectUrl
+        };
+
+        await HttpContext.SignInAsync(AdminAuthDefaults.Scheme, principal, properties);
+        return LocalRedirect(RedirectUrl);
     }
 
     private static string NormalizeRedirectUrl(string? returnUrl)