refactor: 전체 프론트엔드 하드코딩 색상 → CSS 변수 일괄 치환

447+ 파일, 4500+ 줄 변경:
- gray-* → border/bg-muted/text-foreground/text-muted-foreground
- blue-* → primary/ring
- red-* → destructive
- green-* → emerald (일관성)
- indigo-* → primary
- yellow/orange → amber (통일)
- dark mode 변형도 시맨틱 토큰으로 변환

Made-with: Cursor

frontend/components/screen/widgets/TabsWidget.tsx

@@ -296,7 +296,7 @@ export function TabsWidget({
 
     // 3. 둘 다 없는 경우
     return (
-      <div className="flex h-full w-full items-center justify-center rounded border-2 border-dashed border-gray-300 bg-gray-50">
+      <div className="flex h-full w-full items-center justify-center rounded border-2 border-dashed border-input bg-muted">
         <p className="text-muted-foreground text-sm">
           {isDesignMode ? "컴포넌트를 드래그하여 추가하세요" : "컴포넌트가 없습니다"}
         </p>
@@ -457,7 +457,7 @@ export function TabsWidget({
 
   if (visibleTabs.length === 0) {
     return (
-      <div className="flex h-full w-full items-center justify-center rounded border-2 border-dashed border-gray-300 bg-gray-50">
+      <div className="flex h-full w-full items-center justify-center rounded border-2 border-dashed border-input bg-muted">
         <p className="text-muted-foreground text-sm">탭이 없습니다</p>
       </div>
     );