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/css/admin.css

@@ -1370,3 +1370,61 @@ textarea:focus-visible {
     break-inside: avoid;
   }
 }
+
+/* ============================================================================
+   Blazor Loading Indicator
+   ============================================================================ */
+
+#blazor-loading {
+  display: none;
+  position: fixed;
+  top: 0;
+  left: 0;
+  width: 100%;
+  height: 100%;
+  background: rgba(255, 255, 255, 0.95);
+  backdrop-filter: blur(4px);
+  z-index: 9999;
+  align-items: center;
+  justify-content: center;
+}
+
+#blazor-loading.show {
+  display: flex;
+}
+
+.blazor-loading-overlay {
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  width: 100%;
+  height: 100%;
+}
+
+.blazor-loading-spinner {
+  text-align: center;
+  display: flex;
+  flex-direction: column;
+  align-items: center;
+  gap: 1.5rem;
+}
+
+.blazor-loading-spinner p {
+  margin: 0;
+  font-size: 1rem;
+  color: var(--text-primary);
+  font-weight: 500;
+}
+
+.spinner {
+  width: 48px;
+  height: 48px;
+  border: 4px solid var(--bg-tertiary);
+  border-top-color: var(--primary-color);
+  border-radius: 50%;
+  animation: spin 1s linear infinite;
+}
+
+@keyframes spin {
+  to { transform: rotate(360deg); }
+}