feat: 프롬프트 입력창을 Univer 위 오버레이로 변경 및 기본적으로 항상 표시되도록 개선\n\n- 프롬프트 입력창을 Univer 시트 위에 반투명 오버레이로 구현\n- 우하단 플로팅 버튼으로 토글 가능 (기본값: 항상 표시)\n- 입력창 디자인을 컴팩트하게 개선, 반응형 오버레이 적용\n- 기존 하단 고정 방식 제거, Univer 전체 화면 활용\n- 사용자 경험 및 접근성 향상

src/components/sheet/PromptInput.tsx

@@ -1,6 +1,7 @@
 import React, { useEffect, useRef, useState } from "react";
 import { useAppStore } from "../../stores/useAppStore";
 import { aiProcessor } from "../../utils/aiProcessor";
+import { Button } from "../ui/button";
 
 interface PromptInputProps {
   value: string;
@@ -176,45 +177,43 @@ const PromptInput: React.FC<PromptInputProps> = ({
   }, [cellAddressToInsert, onChange, setCellAddressToInsert]);
 
   return (
-    <div className="w-[60%] mx-auto bg-white z-10 flex flex-col items-center py-4 px-2">
-      <div className="w-full max-w-3xl flex items-end gap-2">
+    <div className="w-full z-10 flex flex-col items-center">
+      <div className="w-full flex items-end gap-3">
         <textarea
           ref={textareaRef}
-          className="flex-1 resize-none rounded-lg border border-gray-300 bg-gray-50 px-4 py-3 text-base text-gray-900 focus:outline-none focus:ring-2 focus:ring-blue-400 disabled:bg-gray-100 disabled:cursor-not-allowed min-h-[48px] max-h-32 shadow-sm"
-          placeholder="질문을 입력하세요.
-예시) A1부터 A10까지 합계를 구해서 B1에 입력하는 수식을 입력해줘
-
-💡 팁: 시트에서 셀을 선택하면 자동으로 주소가 입력됩니다"
+          className="flex-1 resize-none rounded-xl border border-gray-300 bg-white px-4 py-3 text-sm text-gray-900 focus:outline-none focus:ring-2 focus:ring-blue-400 disabled:bg-gray-100 disabled:cursor-not-allowed min-h-[44px] max-h-28 shadow-sm"
+          placeholder="AI에게 명령하세요...
+예: A1부터 A10까지 합계를 B1에 입력해줘"
           value={value}
           onChange={onChange}
           disabled={isProcessing}
           maxLength={maxLength}
-          rows={5}
+          rows={3}
         />
         <div style={{ width: "1rem" }} />
 
-        {/* 버튼들을 세로로 배치 */}
-        <div className="flex flex-col gap-3">
-          {/* 히스토리 버튼 - 맨 위 */}
+        {/* 버튼들을 세로로 배치 - 오버레이에 맞게 컴팩트 */}
+        <div className="flex flex-col gap-2">
+          {/* 히스토리 버튼 */}
           {onHistoryToggle && (
             <button
-              className="px-4 py-2 rounded-lg text-gray-600 border border-gray-300 hover:bg-gray-50 font-semibold text-base shadow transition disabled:opacity-60 disabled:cursor-not-allowed flex items-center justify-center gap-2"
+              className="px-2 py-1.5 text-xs font-medium text-gray-700 bg-white border border-gray-300 rounded-lg shadow-sm hover:bg-gray-50 focus:outline-none focus:ring-2 focus:ring-offset-2 focus:ring-blue-500 transition-colors disabled:opacity-60 disabled:cursor-not-allowed flex items-center justify-center gap-1"
               onClick={onHistoryToggle}
               disabled={isProcessing}
               aria-label="작업 히스토리 보기"
             >
               📝
               {historyCount !== undefined && historyCount > 0 && (
-                <span className="text-xs bg-blue-500 text-white rounded-full px-2 py-0.5 min-w-[20px] h-5 flex items-center justify-center">
+                <span className="text-xs bg-blue-500 text-white rounded-full px-1.5 py-0.5 min-w-[16px] h-4 flex items-center justify-center text-[10px]">
                   {historyCount > 99 ? "99+" : historyCount}
                 </span>
               )}
             </button>
           )}
 
-          {/* UNDO 버튼 - 중간 */}
+          {/* UNDO 버튼 */}
           <button
-            className="px-4 py-2 rounded-lg text-gray-600 border border-gray-300 hover:bg-gray-50 font-semibold text-base shadow transition disabled:opacity-60 disabled:cursor-not-allowed flex items-center justify-center gap-2"
+            className="px-2 py-1.5 text-xs font-medium text-gray-700 bg-white border border-gray-300 rounded-lg shadow-sm hover:bg-gray-50 focus:outline-none focus:ring-2 focus:ring-offset-2 focus:ring-blue-500 transition-colors disabled:opacity-60 disabled:cursor-not-allowed flex items-center justify-center"
             onClick={() => {
               // TODO: UNDO 기능 구현
               console.log("🔄 UNDO 버튼 클릭");
@@ -225,25 +224,25 @@ const PromptInput: React.FC<PromptInputProps> = ({
             ↶
           </button>
 
-          {/* 전송하기 버튼 - 맨 아래 */}
-          <button
-            className="px-6 py-2 rounded-lg text-white font-semibold text-base shadow transition disabled:opacity-60 disabled:cursor-not-allowed"
-            style={{
-              background: isProcessing
-                ? "#6b7280"
-                : "linear-gradient(90deg, #a18fff 0%, #6f6fff 100%)",
-            }}
+          {/* 전송하기 버튼 */}
+          <Button
+            variant="outline"
+            size="sm"
+            className={`text-xs px-3 py-1.5 ${
+              isProcessing || !value.trim()
+                ? "bg-gray-400 text-white cursor-not-allowed border-gray-400"
+                : "bg-green-500 hover:bg-green-600 text-white border-green-500"
+            }`}
             onClick={handleExecute}
             disabled={isProcessing || !value.trim()}
           >
-            {isProcessing ? "처리 중..." : "전송하기"}
-          </button>
+            {isProcessing ? "처리중" : "전송"}
+          </Button>
         </div>
       </div>
-      <div className="w-full max-w-3xl flex justify-between items-center mt-1 px-1">
+      <div className="w-full flex justify-between items-center mt-2 px-1">
         <span className="text-xs text-gray-500">
-          Press Enter to send, Shift+Enter for new line | 시트에서 셀 클릭하여
-          주소 자동 입력
+          시트에서 셀 클릭시 자동 입력 | Enter로 전송
         </span>
         <span className="text-xs text-gray-400">
           {value.length}/{maxLength}