refactor: Phase 5 - JWT token lifecycle (Access + Refresh + Auto-refresh)

**Implementation:**
- AuthService: Split token generation
  * AccessToken: 15 minutes
  * RefreshToken: 7 days (10080 minutes)
  * New: GenerateTokenPair() method
  * New: RefreshAccessTokenAsync() method

- AuthTokenPair: New record (accessToken, refreshToken, expiresIn)

- AuthController: New /api/auth/refresh endpoint
  * POST /api/auth/refresh?refreshToken=...
  * Response: { accessToken, refreshToken, expiresIn }
  * RefreshTokenRequest DTO

- TokenRefreshHandler: New DelegatingHandler
  * Automatic Bearer token injection
  * 401 response handling
  * Auto-refresh with retry
  * localStorage sync (accessToken, refreshToken, tokenExpiry)

- CustomAuthenticationStateProvider: Token storage split
  * Before: auth_token (single)
  * After: accessToken, refreshToken, tokenExpiry
  * LoginAsync signature updated

- Login.razor: Handle token pair
  * LoginResponse: { accessToken, refreshToken, expiresIn }
  * Call new LoginAsync(accessToken, refreshToken, expiresIn)

- Program.cs: TokenRefreshHandler registration
  * AddScoped<TokenRefreshHandler>()
  * AdminDashboardClient pipeline: .AddHttpMessageHandler<TokenRefreshHandler>()

**SOLID Principles:**
✓ S (Single Responsibility): TokenRefreshHandler handles only token refresh
✓ D (Dependency Inversion): DelegatingHandler abstracts HTTP concerns
✓ O (Open/Closed): Token lifetime extension without code changes

**Security Pattern:**
- Short-lived access tokens (15min) reduce theft window
- Refresh tokens (7d) enable persistence without storing secrets
- Automatic refresh is transparent to components

**Flow:**
Blazor → AdminDashboardClient → TokenRefreshHandler (auto-add Bearer)
  → 401 → RefreshTokenAsync() → POST /api/auth/refresh
  → Store new pair → Retry original request

Status: Token lifecycle complete, ready for SignalR integration (Phase 6)

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

TaxBaik.Web/Program.cs

@@ -66,9 +66,11 @@ builder.Services.AddCascadingAuthenticationState();
 builder.Services.AddAuthorization();
 builder.Services.AddAuthorizationCore();
 
-// HTTP Client for API
+// HTTP Client for API (with automatic token refresh)
+builder.Services.AddScoped<TokenRefreshHandler>();
 builder.Services.AddHttpClient<IApiClient, ApiClient>();
-builder.Services.AddHttpClient<IAdminDashboardClient, AdminDashboardClient>();
+builder.Services.AddHttpClient<IAdminDashboardClient, AdminDashboardClient>()
+    .AddHttpMessageHandler<TokenRefreshHandler>();
 
 // UI & 캐시
 builder.Services.AddMudServices();