feature: implement persistent login username and remember-me checkbox

Problem: Login form showed remembered username from localStorage, but didn't
restore the 'remember me' checkbox state. Users had to re-check the box on
each login attempt, defeating the purpose of the remember feature.

Solution:
1. AdminLoginForm: Add isRememberChecked field and RememberedCheckboxKey constant
2. OnInitializedAsync: Restore both username AND checkbox state from localStorage
3. admin-session.js bindLoginForm: Restore checkbox.checked from localStorage
4. admin-session.js submit handler: Save checkbox state alongside username

Result: Complete round-trip persistence - when user checks 'remember me' and
logs in, both username and checkbox state persist until explicitly cleared.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

src/TaxBaik.Web.Client/Components/Admin/Shared/AdminLoginForm.razor

@@ -40,18 +40,23 @@
 
 @code {
     private string rememberedUsername = "";
+    private bool isRememberChecked = false;
     private bool isReady;
     private const string RememberedUsernameKey = "admin-remembered-username";
+    private const string RememberedCheckboxKey = "admin-remember-checkbox";
 
     protected override async Task OnInitializedAsync()
     {
         try
         {
             rememberedUsername = await LocalStorageService.GetItemAsStringAsync(RememberedUsernameKey) ?? "";
+            var checkboxValue = await LocalStorageService.GetItemAsStringAsync(RememberedCheckboxKey) ?? "false";
+            isRememberChecked = checkboxValue == "true" && !string.IsNullOrEmpty(rememberedUsername);
         }
         catch
         {
             rememberedUsername = "";
+            isRememberChecked = false;
         }
     }