fix: Select 컴포넌트 빈 문자열 값 오류 수정

- Radix UI Select는 빈 문자열 value를 허용하지 않음
- "선택 안 함" 옵션의 값을 "" → "none"으로 변경
- onValueChange에서 "none" 체크하여 screenId를 null로 설정

frontend/lib/registry/components/conditional-container/ConditionalContainerConfigPanel.tsx

@@ -222,23 +222,30 @@ export function ConditionalContainerConfigPanel({
                       </div>
                     ) : (
                       <Select
-                        value={section.screenId?.toString() || ""}
+                        value={section.screenId?.toString() || "none"}
                         onValueChange={(value) => {
-                          const screenId = value ? parseInt(value) : null;
-                          const selectedScreen = screens.find(
-                            (s) => s.screenId === screenId
-                          );
-                          updateSection(section.id, {
-                            screenId,
-                            screenName: selectedScreen?.screenName,
-                          });
+                          if (value === "none") {
+                            updateSection(section.id, {
+                              screenId: null,
+                              screenName: undefined,
+                            });
+                          } else {
+                            const screenId = parseInt(value);
+                            const selectedScreen = screens.find(
+                              (s) => s.screenId === screenId
+                            );
+                            updateSection(section.id, {
+                              screenId,
+                              screenName: selectedScreen?.screenName,
+                            });
+                          }
                         }}
                       >
                         <SelectTrigger className="h-7 text-xs">
                           <SelectValue placeholder="화면 선택..." />
                         </SelectTrigger>
                         <SelectContent>
-                          <SelectItem value="">선택 안 함</SelectItem>
+                          <SelectItem value="none">선택 안 함</SelectItem>
                           {screens.map((screen) => (
                             <SelectItem
                               key={screen.screenId}