refactor: Phase 4 - Dashboard Blazor → API client (Service Locator → Dependency Injection)

**Implementation:**
- AdminDashboardClient: HTTP API client interface
  - GetSummaryAsync: Fetch dashboard metrics
  - GetUpcomingFilingsAsync: 30-day filings forecast
  - GetRecentInquiriesAsync: Latest inquiries
  - GetMonthlyStatsAsync: Monthly statistics
- Program.cs: Register IAdminDashboardClient
- Dashboard.razor: Replace service injection with API client
  - Remove: Direct AdminDashboardService/TaxFilingService injection
  - Add: IAdminDashboardClient injection
  - Add: Error handling & loading state
  - Change: OnInitializedAsync() calls API endpoints

**SOLID Principles Applied:**
✓ D (Dependency Inversion): Blazor depends on IAdminDashboardClient abstraction
✓ S (Single Responsibility): Client handles only HTTP communication
✓ O (Open/Closed): Can extend API without changing Blazor component

**Architecture Pattern:**
- Before: Blazor → Service (server-side logic) → Repository → DB
- After: Blazor → HTTP → API → Service → Repository → DB

**Benefits:**
- Clear separation of concerns
- Easier to test (mock HTTP)
- Foundation for token refresh middleware
- Prepare for SignalR integration

Status: Ready for Phase 5 (JWT token refresh)
Next: Implement automatic token refresh on 401 responses

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

TaxBaik.Web/Program.cs

@@ -68,6 +68,7 @@ builder.Services.AddAuthorizationCore();
 
 // HTTP Client for API
 builder.Services.AddHttpClient<IApiClient, ApiClient>();
+builder.Services.AddHttpClient<IAdminDashboardClient, AdminDashboardClient>();
 
 // UI & 캐시
 builder.Services.AddMudServices();