feat: add admin username remember functionality to login page

- Add 'Remember ID' checkbox for improved UX
- Store username in localStorage when checked
- Restore saved username on login page load
- Remove saved username when checkbox unchecked
- Follow security best practice: save username only, not password

TaxBaik.Web/Components/Admin/Pages/Login.razor

@@ -6,6 +6,7 @@
 @inject NavigationManager NavigationManager
 @inject CustomAuthenticationStateProvider AuthStateProvider
 @inject IJSRuntime Js
+@inject ILocalStorageService LocalStorageService
 
 <PageTitle>로그인</PageTitle>
 
@@ -27,6 +28,11 @@
                        autocomplete="current-password"
                        @bind-Value="model.Password" />
 
+            <div class="mb-4">
+                <InputCheckbox class="mud-checkbox" @bind-Value="model.RememberMe" />
+                <label style="margin-left: 8px; cursor: pointer;">아이디 저장</label>
+            </div>
+
             @if (!string.IsNullOrEmpty(errorMessage))
             {
                 <MudAlert Severity="Severity.Error" Class="mb-4">@errorMessage</MudAlert>
@@ -53,8 +59,25 @@
 @code {
     private bool isLoading = false;
     private string errorMessage = "";
-
     private LoginModel model = new();
+    private const string RememberedUsernameKey = "admin-remembered-username";
+
+    protected override async Task OnInitializedAsync()
+    {
+        try
+        {
+            var remembered = await LocalStorageService.GetItemAsStringAsync(RememberedUsernameKey);
+            if (!string.IsNullOrEmpty(remembered))
+            {
+                model.Username = remembered;
+                model.RememberMe = true;
+            }
+        }
+        catch
+        {
+            // LocalStorage not available in pre-render
+        }
+    }
 
     protected override async Task OnAfterRenderAsync(bool firstRender)
     {
@@ -82,6 +105,15 @@
                 return;
             }
 
+            if (model.RememberMe)
+            {
+                await LocalStorageService.SetItemAsStringAsync(RememberedUsernameKey, model.Username);
+            }
+            else
+            {
+                await LocalStorageService.RemoveItemAsync(RememberedUsernameKey);
+            }
+
             await ApiClient.SetAuthToken(response.AccessToken);
             await AuthStateProvider.LoginAsync(response.AccessToken, response.RefreshToken, response.ExpiresIn);
             NavigationManager.NavigateTo(GetReturnUrl(), forceLoad: false);
@@ -104,6 +136,7 @@
     {
         public string Username { get; set; } = "";
         public string Password { get; set; } = "";
+        public bool RememberMe { get; set; }
     }
 
     private string GetReturnUrl()