fix: check cached page state in showLoading() before starting MutationObserver

- Page may be already rendered when showLoading() is called (fast nav, cached state)
- Check .admin-page-hero / .admin-login-page immediately and hide if present
- Prevents stuck loading overlay on rapid navigation between pages

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

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

@@ -17,7 +17,20 @@ window.taxbaikAdminSession = {
         const overlay = document.getElementById('blazor-loading');
         if (!overlay) return;
 
-        // Start observer FIRST so it catches the mutation that brings new content in.
+        // Show overlay immediately
+        overlay.classList.add('show');
+
+        // Check if page is already ready (cached state on fast nav)
+        const pageReady =
+            document.querySelector('.admin-page-hero') !== null ||
+            document.querySelector('.admin-login-page') !== null;
+        if (pageReady) {
+            // Page already rendered, hide immediately
+            window.taxbaikAdminSession.hideLoading();
+            return;
+        }
+
+        // Start observer to catch future mutations
         if (window._taxbaikLoadingObserver) {
             window._taxbaikLoadingObserver.disconnect();
         }
@@ -34,9 +47,6 @@ window.taxbaikAdminSession = {
             subtree: true
         });
 
-        // Show overlay after observer is active.
-        overlay.classList.add('show');
-
         // Safety fallback: hide after 3 seconds regardless.
         if (window._taxbaikLoadingTimeout) {
             clearTimeout(window._taxbaikLoadingTimeout);