feat(pop): POP 화면 관리 시스템 구현

Backend:
- screen_layouts_pop 테이블용 CRUD API 추가 (getLayoutPop, saveLayoutPop, deleteLayoutPop, getScreenIdsWithPopLayout)
- 멀티테넌시 권한 체크 포함

Frontend API:
- screenApi에 POP 레이아웃 함수 4개 추가

POP 관리 페이지:
- popScreenMngList 신규 생성
- isPop prop으로 미리보기 URL 분기 (/pop/screens/{id})
- CreateScreenModal에서 POP 화면 생성 시 빈 레이아웃 자동 생성

POP 디자이너:
- PopDesigner, PopCanvas, PopPanel, SectionGrid 컴포넌트 구현
- react-dnd로 팔레트→캔버스 드래그앤드롭
- react-grid-layout으로 컴포넌트 자유 배치/리사이즈
- 그리드 단순화: 고정 셀 크기(40px) 기반 자동 계산, 그리드 점 제거
- onLayoutChange를 onDragStop/onResizeStop으로 변경하여 드롭 시 크기 유지

frontend/components/ui/resizable.tsx

@@ -0,0 +1,45 @@
+"use client";
+
+import { GripVertical } from "lucide-react";
+import * as ResizablePrimitive from "react-resizable-panels";
+
+import { cn } from "@/lib/utils";
+
+const ResizablePanelGroup = ({
+  className,
+  ...props
+}: React.ComponentProps<typeof ResizablePrimitive.PanelGroup>) => (
+  <ResizablePrimitive.PanelGroup
+    className={cn(
+      "flex h-full w-full data-[panel-group-direction=vertical]:flex-col",
+      className
+    )}
+    {...props}
+  />
+);
+
+const ResizablePanel = ResizablePrimitive.Panel;
+
+const ResizableHandle = ({
+  withHandle,
+  className,
+  ...props
+}: React.ComponentProps<typeof ResizablePrimitive.PanelResizeHandle> & {
+  withHandle?: boolean;
+}) => (
+  <ResizablePrimitive.PanelResizeHandle
+    className={cn(
+      "relative flex w-px items-center justify-center bg-border after:absolute after:inset-y-0 after:left-1/2 after:w-1 after:-translate-x-1/2 focus-visible:outline-none focus-visible:ring-1 focus-visible:ring-ring focus-visible:ring-offset-1 data-[panel-group-direction=vertical]:h-px data-[panel-group-direction=vertical]:w-full data-[panel-group-direction=vertical]:after:left-0 data-[panel-group-direction=vertical]:after:h-1 data-[panel-group-direction=vertical]:after:w-full data-[panel-group-direction=vertical]:after:-translate-y-1/2 data-[panel-group-direction=vertical]:after:translate-x-0 [&[data-panel-group-direction=vertical]>div]:rotate-90",
+      className
+    )}
+    {...props}
+  >
+    {withHandle && (
+      <div className="z-10 flex h-4 w-3 items-center justify-center rounded-sm border bg-border">
+        <GripVertical className="h-2.5 w-2.5" />
+      </div>
+    )}
+  </ResizablePrimitive.PanelResizeHandle>
+);
+
+export { ResizablePanelGroup, ResizablePanel, ResizableHandle };