fix: hide modal/player overlays when hidden attr is set

.modalOverlay { display: flex } and .playerOverlay { display: flex }
were overriding the browser default [hidden] { display: none }, so the
"폴더 추가" modal stayed visible on /op/dashboard load and blocked all
other UI. Added a single [hidden] { display: none !important } rule so
the hidden attribute always wins regardless of later display rules.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

public/styles.css

@@ -14,6 +14,8 @@
 
 * { box-sizing: border-box; }
 html, body { margin: 0; padding: 0; }
+/* display: flex 등이 걸린 오버레이/모달이라도 hidden 속성을 항상 우선시킨다. */
+[hidden] { display: none !important; }
 
 body.siteBody {
   font-family: 'Pretendard', -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, sans-serif;