fix: simplify cookie configuration - remove duplicate Antiforgery setup

- Remove explicit AddAntiforgery (already auto-registered)
- Keep only session cookie with SameSite=Lax
- Rely on UseForwardedHeaders for proxy HTTPS detection

ASP.NET Core automatically registers Antiforgery, so explicit
configuration causes duplicate setup. Simplified to essential
cookie settings only.

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

src/TaxBaik.Web/Program.cs

@@ -107,18 +107,8 @@ builder.Services.AddSession(options =>
     options.Cookie.IsEssential = true;
     options.Cookie.Name = "TaxBaik.SessionId";
     options.Cookie.SameSite = Microsoft.AspNetCore.Http.SameSiteMode.Lax;
-    options.Cookie.SecurePolicy = Microsoft.AspNetCore.Http.CookieSecurePolicy.SameAsRequest;
 });
 builder.Services.AddDistributedMemoryCache();
-
-// Antiforgery 쿠키 설정 (Nginx 프록시 뒤 HTTPS 지원)
-builder.Services.AddAntiforgery(options =>
-{
-    options.Cookie.HttpOnly = true;
-    options.Cookie.IsEssential = true;
-    options.Cookie.SameSite = Microsoft.AspNetCore.Http.SameSiteMode.Lax;
-    options.Cookie.SecurePolicy = Microsoft.AspNetCore.Http.CookieSecurePolicy.SameAsRequest;
-});
 // TempData는 기본적으로 쿠키 저장소 사용 (위 세션 설정 상속)
 
 // JWT 인증