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/FlowButtonGroup.tsx

@@ -139,12 +139,12 @@ export const FlowButtonGroup: React.FC<FlowButtonGroupProps> = ({
             {renderButton(button, buttonVisibility[index])}
             {!buttonVisibility[index] && (
               <div
-                className="pointer-events-none absolute inset-0 flex items-center justify-center bg-gray-900/10"
+                className="pointer-events-none absolute inset-0 flex items-center justify-center bg-foreground/10"
                 style={{
                   border: "1px dashed #94a3b8",
                 }}
               >
-                <span className="text-[10px] text-gray-500">숨김</span>
+                <span className="text-[10px] text-muted-foreground">숨김</span>
               </div>
             )}
           </div>