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/Components/Admin/App.razor

@@ -24,6 +24,9 @@
             <span>배포 또는 서버 재시작 중이면 잠시 후 자동으로 새로고침됩니다.</span>
         </div>
     </div>
+    <MudThemeProvider @bind-IsDarkMode="isDarkMode" Theme="mudTheme" />
+    <MudDialogProvider />
+    <MudSnackbarProvider />
     <Routes @rendermode="new InteractiveServerRenderMode(prerender: false)" />
     <script src="_content/MudBlazor/MudBlazor.min.js"></script>
     <script src="js/admin-session.js"></script>
@@ -31,3 +34,90 @@
     <script>window.taxbaikAdminSession?.watchReconnect();</script>
 </body>
 </html>
+
+@code {
+    private bool isDarkMode = false;
+    private MudTheme mudTheme = new()
+    {
+        Palette = new PaletteLight()
+        {
+            Primary = "#1976D2",
+            PrimaryContrastText = "#FFFFFF",
+            Secondary = "#2D9F7E",
+            SecondaryContrastText = "#FFFFFF",
+            Tertiary = "#FF8A50",
+            TertiaryContrastText = "#FFFFFF",
+            Surface = "#F5F7FA",
+            Background = "#FFFFFF",
+            BackgroundGrey = "#F8F9FB",
+            DrawerBackground = "#FFFFFF",
+            DrawerText = "#424242",
+            AppbarBackground = "#FFFFFF",
+            AppbarText = "#424242",
+            TextPrimary = "#1A1A1A",
+            TextSecondary = "#64748B",
+            TextDisabled = "#94A3B8",
+            ActionDefault = "#1976D2",
+            ActionDisabled = "#BDBDBD",
+            Divider = "#E2E8F0",
+            DividerLight = "#F1F5F9",
+            Error = "#DC2626",
+            ErrorContrastText = "#FFFFFF",
+            Warning = "#F59E0B",
+            WarningContrastText = "#FFFFFF",
+            Info = "#06B6D4",
+            InfoContrastText = "#FFFFFF",
+            Success = "#16A34A",
+            SuccessContrastText = "#FFFFFF",
+        },
+        LayoutProperties = new LayoutProperties()
+        {
+            DefaultBorderRadius = "8px"
+        },
+        Typography = new Typography()
+        {
+            Default = new Default()
+            {
+                FontSize = ".875rem",
+                FontWeight = 400,
+                LineHeight = 1.5
+            },
+            H1 = new H1()
+            {
+                FontSize = "2.5rem",
+                FontWeight = 600,
+                LineHeight = 1.2
+            },
+            H2 = new H2()
+            {
+                FontSize = "2rem",
+                FontWeight = 600,
+                LineHeight = 1.3
+            },
+            H3 = new H3()
+            {
+                FontSize = "1.75rem",
+                FontWeight = 600,
+                LineHeight = 1.3
+            },
+            H4 = new H4()
+            {
+                FontSize = "1.5rem",
+                FontWeight = 600,
+                LineHeight = 1.4
+            },
+            H5 = new H5()
+            {
+                FontSize = "1.25rem",
+                FontWeight = 500,
+                LineHeight = 1.4
+            },
+            H6 = new H6()
+            {
+                FontSize = "1rem",
+                FontWeight = 500,
+                LineHeight = 1.5
+            }
+        }
+    };
+}

TaxBaik.Web/Program.cs

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