[RAPID-fix] 캡처 속도/화질 개선 + 드래그 커서 4방향 화살표로 변경

- 오버레이 마운트 시점에 미리 캡처 시작 → mouseup 즉시 크롭
- scale: max(dpr, 2)로 화질 2배 향상
- 캡처 준비 중 wait 커서 표시
- 메신저 헤더 드래그 커서 cursor-grab → cursor-move (4방향 화살표)

frontend/components/messenger/MessengerModal.tsx

@@ -189,7 +189,7 @@ export function MessengerModal() {
 
       {/* Header */}
       <div
-        className="flex items-center justify-between px-4 py-2 border-b bg-muted/30 select-none cursor-grab active:cursor-grabbing"
+        className="flex items-center justify-between px-4 py-2 border-b bg-muted/30 select-none cursor-move"
         onMouseDown={onHeaderMouseDown}
       >
         <h2 className="text-sm font-semibold">메신저</h2>

frontend/components/messenger/ScreenCapture.tsx

@@ -16,10 +16,37 @@ interface ScreenCaptureProps {
 
 export function ScreenCapture({ onCapture, onCancel }: ScreenCaptureProps) {
   const canvasRef = useRef<HTMLCanvasElement>(null);
+  const capturedImgRef = useRef<HTMLImageElement | null>(null);
+  const [ready, setReady] = useState(false);
   const [selecting, setSelecting] = useState(false);
   const startRef = useRef<{ x: number; y: number } | null>(null);
   const [rect, setRect] = useState<Rect | null>(null);
 
+  // Pre-capture on mount so mouseup is instant
+  useEffect(() => {
+    let cancelled = false;
+    (async () => {
+      try {
+        const { domToPng } = await import("modern-screenshot");
+        const scale = Math.max(window.devicePixelRatio || 1, 2);
+        const dataUrl = await domToPng(document.body, {
+          width: window.innerWidth,
+          height: window.innerHeight,
+          scale,
+        });
+        if (cancelled) return;
+        const img = new Image();
+        img.src = dataUrl;
+        await new Promise((res) => { img.onload = res; });
+        capturedImgRef.current = img;
+        setReady(true);
+      } catch {
+        if (!cancelled) onCancel();
+      }
+    })();
+    return () => { cancelled = true; };
+  }, [onCancel]);
+
   // ESC to cancel
   useEffect(() => {
     const handler = (e: KeyboardEvent) => {
@@ -47,70 +74,51 @@ export function ScreenCapture({ onCapture, onCancel }: ScreenCaptureProps) {
     setRect(getRect(startRef.current.x, startRef.current.y, e.clientX, e.clientY));
   };
 
-  const handleMouseUp = async (e: React.MouseEvent) => {
+  const handleMouseUp = (e: React.MouseEvent) => {
     if (!selecting || !startRef.current) return;
     setSelecting(false);
     const r = getRect(startRef.current.x, startRef.current.y, e.clientX, e.clientY);
     startRef.current = null;
 
-    if (r.w < 4 || r.h < 4) {
-      onCancel();
-      return;
-    }
+    if (r.w < 4 || r.h < 4) { onCancel(); return; }
 
-    // Capture via modern-screenshot
-    try {
-      const { domToPng } = await import("modern-screenshot");
-      const dataUrl = await domToPng(document.body, {
-        width: window.innerWidth,
-        height: window.innerHeight,
-      });
+    const img = capturedImgRef.current;
+    if (!img) { onCancel(); return; }
 
-      // Crop the captured region — image is at CSS pixel scale
-      const img = new Image();
-      img.src = dataUrl;
-      await new Promise((res) => { img.onload = res; });
+    const scaleX = img.naturalWidth / window.innerWidth;
+    const scaleY = img.naturalHeight / window.innerHeight;
 
-      // Scale factor between actual image size and CSS pixels
-      const scaleX = img.naturalWidth / window.innerWidth;
-      const scaleY = img.naturalHeight / window.innerHeight;
+    const canvas = canvasRef.current!;
+    canvas.width = r.w * scaleX;
+    canvas.height = r.h * scaleY;
+    const ctx = canvas.getContext("2d")!;
+    ctx.drawImage(
+      img,
+      r.x * scaleX, r.y * scaleY, r.w * scaleX, r.h * scaleY,
+      0, 0, r.w * scaleX, r.h * scaleY,
+    );
 
-      const canvas = canvasRef.current!;
-      canvas.width = r.w * scaleX;
-      canvas.height = r.h * scaleY;
-      const ctx = canvas.getContext("2d")!;
-      ctx.drawImage(
-        img,
-        r.x * scaleX, r.y * scaleY, r.w * scaleX, r.h * scaleY,
-        0, 0, r.w * scaleX, r.h * scaleY,
-      );
-
-      canvas.toBlob((blob) => {
-        if (!blob) { onCancel(); return; }
-        const file = new File([blob], `capture-${Date.now()}.png`, { type: "image/png" });
-        onCapture(file);
-      }, "image/png");
-    } catch {
-      onCancel();
-    }
+    canvas.toBlob((blob) => {
+      if (!blob) { onCancel(); return; }
+      const file = new File([blob], `capture-${Date.now()}.png`, { type: "image/png" });
+      onCapture(file);
+    }, "image/png");
   };
 
   return (
     <>
       <canvas ref={canvasRef} className="hidden" />
       <div
-        className="fixed inset-0 bg-black/40 cursor-crosshair select-none"
-        style={{ zIndex: 99999 }}
-        onMouseDown={handleMouseDown}
+        className="fixed inset-0 bg-black/40 select-none"
+        style={{ zIndex: 99999, cursor: ready ? "crosshair" : "wait" }}
+        onMouseDown={ready ? handleMouseDown : undefined}
         onMouseMove={handleMouseMove}
         onMouseUp={handleMouseUp}
       >
-        {/* instruction */}
         <div className="absolute top-4 left-1/2 -translate-x-1/2 bg-black/70 text-white text-sm px-4 py-2 rounded-full pointer-events-none">
-          드래그하여 캡처 영역을 선택하세요 &nbsp;·&nbsp; ESC로 취소
+          {ready ? "드래그하여 캡처 영역을 선택하세요" : "캡처 준비 중..."}&nbsp;·&nbsp;ESC로 취소
         </div>
 
-        {/* selection rect */}
         {rect && rect.w > 0 && rect.h > 0 && (
           <div
             className="absolute border-2 border-blue-400 bg-blue-400/10 pointer-events-none"