fix(fe): CommonCodeManagementPage - input reset bug on new row edit

Problem: Adding a group row then typing in cells caused continuous reset
Root Cause: Vue reactivity lost when assigning new array to reactive object
Solution: Use Object.assign() to maintain reactivity tracking

- Changed: mockChildCodesMap[key] = list
- To: Object.assign(mockChildCodesMap, { [key]: list })
- Result: Cell input now persists without reset

Affects: /system/common-codes grid editing

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

frontend/src/features/system/pages/CommonCodeManagementPage.vue

@@ -239,7 +239,7 @@ const addGridRow = () => {
   let list = mockChildCodesMap[groupCodeKey]
   if (!list) {
     list = []
-    mockChildCodesMap[groupCodeKey] = list
+    Object.assign(mockChildCodesMap, { [groupCodeKey]: list })
   }
   const newRow: CommonCode = {
     code: `NEW_CODE_${list.length + 1}`,