style(ui): 배경색 통일 및 스크롤 최적화

- autocomplete-search-input: !bg-background 강제 적용
- section-paper: 배경색 진하게(bg-muted/40), 기본 테두리 활성화, overflow-visible
- modal-repeater-table: tbody 흰색 배경, 스크롤 높이 제한(240px), 헤더 고정
- autocomplete 드롭다운: z-index 100으로 상향

배경색 통일로 일관된 디자인, 스크롤로 공간 효율 개선

frontend/lib/registry/components/modal-repeater-table/RepeaterTable.tsx

@@ -118,10 +118,10 @@ export function RepeaterTable({
   };
 
   return (
-    <div className="border rounded-md overflow-hidden">
-      <div className="overflow-x-auto">
+    <div className="border rounded-md overflow-hidden bg-background">
+      <div className="overflow-x-auto max-h-[240px] overflow-y-auto">
         <table className="w-full text-xs sm:text-sm">
-          <thead className="bg-muted">
+          <thead className="bg-muted sticky top-0 z-10">
             <tr>
               <th className="px-4 py-2 text-left font-medium text-muted-foreground w-12">
                 #
@@ -141,7 +141,7 @@ export function RepeaterTable({
               </th>
             </tr>
           </thead>
-          <tbody>
+          <tbody className="bg-background">
             {data.length === 0 ? (
               <tr>
                 <td