fix(auth): 로그아웃 시 프록시 캐시 쿠키 삭제 속성 일치

삭제 쿠키에 Secure, HttpOnly, SameSite 속성 추가하여
원본 쿠키와 매칭되도록 수정 (gc_proxy_auth, GC_SESSION)

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

src/main/java/com/gcsc/guide/auth/AuthController.java

@@ -227,9 +227,11 @@ public class AuthController {
 
     private void clearSessionCookies(HttpServletResponse response) {
         response.addHeader(HttpHeaders.SET_COOKIE,
-            ResponseCookie.from("GC_SESSION", "").path("/").maxAge(0).build().toString());
+            ResponseCookie.from("GC_SESSION", "")
+                .path("/").httpOnly(true).secure(true).sameSite("Lax").maxAge(0).build().toString());
         response.addHeader(HttpHeaders.SET_COOKIE,
-            ResponseCookie.from("gc_proxy_auth", "").path("/").maxAge(0).build().toString());
+            ResponseCookie.from("gc_proxy_auth", "")
+                .path("/").httpOnly(true).secure(true).sameSite("Lax").maxAge(0).build().toString());
     }
 
     private String getCookieValue(HttpServletRequest request, String name) {