fix: use correct client log method name in login error handler

Problem: Line 350 calls postLog() which is not defined in the login form scope.
postLog is a local variable inside initErrorLogging() and not accessible here.

Solution: Use window.taxbaikAdminSession.postClientLog() instead, which is
the public method created by initErrorLogging() and assigned to the window object.

Result: Login errors are now properly logged without ReferenceError.

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

src/TaxBaik.Web/wwwroot/js/admin-session.js

@@ -347,18 +347,20 @@ window.taxbaikAdminSession = {
                 }, 200);
             } catch (error) {
                 window.taxbaikAdminSession.traceUiState('admin-login', `submit failed: ${error?.message || 'login failed'}`);
-                postLog({
-                    level: 'error',
-                    source: 'admin-login-form',
-                    message: error?.message || 'login failed',
-                    url: window.location.href,
-                    route: window.location.pathname + window.location.search,
-                    buildVersion: window.taxbaikAdminBuildVersion || '',
-                    component: 'AdminLoginForm',
-                    viewportWidth: window.taxbaikAdminSession.getViewportWidth(),
-                    userAgent: navigator.userAgent || '',
-                    stack: error?.stack || ''
-                });
+                if (window.taxbaikAdminSession.postClientLog) {
+                    window.taxbaikAdminSession.postClientLog({
+                        level: 'error',
+                        source: 'admin-login-form',
+                        message: error?.message || 'login failed',
+                        url: window.location.href,
+                        route: window.location.pathname + window.location.search,
+                        buildVersion: window.taxbaikAdminBuildVersion || '',
+                        component: 'AdminLoginForm',
+                        viewportWidth: window.taxbaikAdminSession.getViewportWidth(),
+                        userAgent: navigator.userAgent || '',
+                        stack: error?.stack || ''
+                    });
+                }
                 const errorMessage = document.createElement('div');
                 errorMessage.className = 'mud-alert mud-alert-filled-error login-error-message mb-4';
                 errorMessage.textContent = '로그인 중 오류가 발생했습니다.';