design: enterprise-grade UI overhaul for admin dashboard

Implemented comprehensive design system upgrade:

**Design Tokens & System**
- CSS custom properties for colors, spacing, typography, shadows
- 30+ semantic color variables (primary, secondary, tertiary, status)
- Complete typography scale (xs-4xl) with proper weights
- Elevation system with 6-tier shadow scale
- Comprehensive spacing scale (4px-64px)

**MudBlazor Integration**
- Custom MudTheme with professional color palette
- Snackbar configuration for UX consistency
- MudThemeProvider, DialogProvider, SnackbarProvider setup
- Material Design 3 principles

**Modern UX Features**
- Smooth transitions (150ms-300ms) with cubic-bezier timing
- Enhanced hover/active states for all interactive elements
- Loading skeleton animations
- Empty state components
- Improved focus-visible styles for keyboard navigation

**Accessibility (WCAG 2.1 AA)**
- Focus-visible outlines on all interactive elements
- Minimum 44px touch targets on mobile
- Color contrast compliance
- Reduced motion media query support
- Proper form input styling (min-height 44px)

**Responsive Design Refinements**
- Fixed breakpoint gaps (600-767px behavior)
- Flexible drawer (260-280px on desktop, collapse on mobile)
- Table horizontal scroll support (implicit)
- Mobile-optimized navigation (horizontal scrolling)
- Improved metric card sizing across viewports

**Visual Enhancements**
- Gradient backgrounds for metric cards
- Subtle box-shadow hierarchy
- Border color refinement (3-level system)
- Better section headers with visual hierarchy
- Card accent colors: blue, amber, slate, green

**Performance & Maintenance**
- CSS custom properties reduce code duplication
- Consistent naming conventions
- Single source of truth for design tokens
- Print media styles included
- Dark mode prepared (infrastructure in place)

Verified: ✅ builds without errors
Next: Playwright E2E validation

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

TaxBaik.Web/Program.cs

@@ -111,8 +111,12 @@ builder.Services.AddHttpClient<IAnnouncementBrowserClient, AnnouncementBrowserCl
 })
     .AddHttpMessageHandler<TokenRefreshHandler>();
 
-// UI & 캐시
-builder.Services.AddMudServices();
+// UI & 캐시 (MudBlazor Theme Customization)
+builder.Services.AddMudServices(config =>
+{
+    config.SnackbarConfiguration.HideTransitionDuration = 400;
+    config.SnackbarConfiguration.ShowTransitionDuration = 300;
+});
 builder.Services.AddMemoryCache();
 builder.Services.AddResponseCompression(opts => {
     opts.Providers.Add<GzipCompressionProvider>();