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/wwwroot/js/admin-session.js

@@ -277,6 +277,19 @@ window.taxbaikAdminSession = {
             if (label) label.textContent = '로그인';
         }
 
+        // 체크박스 상태 복원
+        const rememberCheckbox = form.querySelector('input[type="checkbox"]');
+        if (rememberCheckbox) {
+            try {
+                const savedState = localStorage.getItem('admin-remember-checkbox');
+                if (savedState === 'true') {
+                    rememberCheckbox.checked = true;
+                }
+            } catch {
+                // localStorage access error
+            }
+        }
+
         if (form.dataset.bound === '1') return;
         form.dataset.bound = '1';
         window.taxbaikAdminSession.traceUiState('admin-login', 'bindLoginForm attached');
@@ -322,8 +335,10 @@ window.taxbaikAdminSession = {
 
                 if (rememberMe) {
                     localStorage.setItem('admin-remembered-username', username);
+                    localStorage.setItem('admin-remember-checkbox', 'true');
                 } else {
                     localStorage.removeItem('admin-remembered-username');
+                    localStorage.removeItem('admin-remember-checkbox');
                 }
 
                 window.location.href = '/taxbaik/admin/dashboard';