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/Components/Admin/App.razor

@@ -24,6 +24,12 @@
             <span>배포 또는 서버 재시작 중이면 잠시 후 자동으로 새로고침됩니다.</span>
         </div>
     </div>
+    <div id="blazor-loading" class="blazor-loading-overlay">
+        <div class="blazor-loading-spinner">
+            <div class="spinner"></div>
+            <p>로드 중...</p>
+        </div>
+    </div>
     <MudThemeProvider @bind-IsDarkMode="isDarkMode" Theme="mudTheme" />
     <MudDialogProvider />
     <MudSnackbarProvider />