feat: add detailed logging to diagnose login redirect flow

Added trace logging to admin-session.js to track form submission:
- Log when username/password fields are detected
- Helps identify where submission might be failing

Status: Login flow confirmed working in local tests
- Username/password correctly extracted from form fields
- localStorage token successfully stored
- Dashboard redirect verified (URL confirmed)

Next: Validate in production environment

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

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

@@ -308,6 +308,8 @@ window.taxbaikAdminSession = {
             if (submitButton) submitButton.disabled = true;
 
             try {
+                window.taxbaikAdminSession.traceUiState('admin-login', `submit: username=${username ? 'yes' : 'no'}, password=${password ? 'yes' : 'no'}`);
+
                 if (!username || !password) {
                     throw new Error('username/password missing');
                 }