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/autocomplete-search-input/AutocompleteSearchInputComponent.tsx

@@ -175,7 +175,7 @@ export function AutocompleteSearchInputComponent({
           onFocus={handleInputFocus}
           placeholder={placeholder}
           disabled={disabled}
-          className="h-8 pr-16 text-xs sm:h-10 sm:text-sm"
+          className="h-8 pr-16 text-xs sm:h-10 sm:text-sm !bg-background"
         />
         <div className="absolute right-1 top-1/2 flex -translate-y-1/2 items-center gap-1">
           {loading && (
@@ -198,7 +198,7 @@ export function AutocompleteSearchInputComponent({
 
       {/* 드롭다운 결과 */}
       {isOpen && (results.length > 0 || loading) && (
-        <div className="absolute z-50 mt-1 max-h-[300px] w-full overflow-y-auto rounded-md border bg-background shadow-lg">
+        <div className="absolute z-[100] mt-1 max-h-[300px] w-full overflow-y-auto rounded-md border bg-background shadow-lg">
           {loading && results.length === 0 ? (
             <div className="p-4 text-center text-sm text-muted-foreground">
               <Loader2 className="mx-auto mb-2 h-4 w-4 animate-spin" />