feat: add blazor loading indicator during page transitions

**Issue**: White screen appears 1-2 seconds during page load/transitions while Blazor circuit connects

**Solution**: Add loading spinner overlay that displays while Blazor initializes

**Changes**:
1. App.razor: Add loading overlay HTML element
2. admin.css: Add loading spinner styles + animations
3. admin-session.js: Show overlay on load, hide when Blazor circuit ready

**UX Flow**:
- Page load starts → Blazor loading spinner appears
- Blazor circuit connects (~1-2s) → Spinner disappears
- Page fully interactive → User sees content

**Styling**:
- Centered spinner with 'Loading...' text
- Semi-transparent background (blur effect)
- Smooth fade-out when complete
- High z-index (9999) to cover all content

This provides clear visual feedback that the app is working, not frozen.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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

@@ -15,6 +15,18 @@ window.taxbaikAdminSession = {
         window.taxbaikAdminSession.syncRouteClass();
         window.addEventListener('popstate', window.taxbaikAdminSession.syncRouteClass);
 
+        // Show loading indicator on page load (hide when Blazor circuit connects)
+        const loadingOverlay = document.getElementById('blazor-loading');
+        if (loadingOverlay) {
+            loadingOverlay.classList.add('show');
+            // Hide loading when Blazor is ready
+            Blazor.start().then(() => {
+                if (loadingOverlay) {
+                    loadingOverlay.classList.remove('show');
+                }
+            });
+        }
+
         const modal = document.getElementById('components-reconnect-modal');
         if (!modal) {
             return;