feat: NavMenu 로그아웃 버튼 + 토큰 갱신 로직

1. NavMenu에 사용자명 표시 및 로그아웃 버튼 추가
2. CustomAuthenticationStateProvider에 토큰 만료 검증 추가
3. Routes.razor 간소화 (AuthorizeRouteView 사용)
4. 미인증 사용자는 _Imports.razor의 [Authorize]로 보호됨

테스트 계정: admin / admin123

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

TaxBaik.Admin/Services/CustomAuthenticationStateProvider.cs

@@ -1,3 +1,4 @@
+using System.IdentityModel.Tokens.Jwt;
 using System.Security.Claims;
 using Microsoft.AspNetCore.Components.Authorization;
 
@@ -27,6 +28,13 @@ public class CustomAuthenticationStateProvider : AuthenticationStateProvider
                 return new AuthenticationState(new ClaimsPrincipal(new ClaimsIdentity()));
             }
 
+            if (IsTokenExpired(token))
+            {
+                _logger.LogWarning("토큰 만료됨");
+                await _localStorage.RemoveItemAsync("auth_token");
+                return new AuthenticationState(new ClaimsPrincipal(new ClaimsIdentity()));
+            }
+
             var principal = _authService.ValidateToken(token);
             if (principal == null)
             {
@@ -46,8 +54,6 @@ public class CustomAuthenticationStateProvider : AuthenticationStateProvider
     public async Task LoginAsync(string token)
     {
         await _localStorage.SetItemAsStringAsync("auth_token", token);
-
-        var principal = _authService.ValidateToken(token);
         NotifyAuthenticationStateChanged(GetAuthenticationStateAsync());
     }
 
@@ -56,4 +62,18 @@ public class CustomAuthenticationStateProvider : AuthenticationStateProvider
         await _localStorage.RemoveItemAsync("auth_token");
         NotifyAuthenticationStateChanged(GetAuthenticationStateAsync());
     }
+
+    private bool IsTokenExpired(string token)
+    {
+        try
+        {
+            var handler = new JwtSecurityTokenHandler();
+            var jwtToken = handler.ReadJwtToken(token);
+            return jwtToken.ValidTo < DateTime.UtcNow;
+        }
+        catch
+        {
+            return true;
+        }
+    }
 }