IP 등록 UI 및 검증 로직 업데이트

- `.ip-input-row`/`.btn-add-ip` 정렬 및 버튼 크기 수정
- Callback URL 관련 입력 및 검증 로직 주석 처리
- 화이트리스트 필수 입력 항목 검증 로직 추가

src/main/resources/static/sass/pages/_apikey-register.scss

@@ -370,6 +370,7 @@
 // IP Input Row - Step 1 Style
 .ip-input-row {
   display: flex;
+  align-items: center;
   gap: 21px;
 
   .ip-input {
@@ -398,7 +399,7 @@
 }
 
 .btn-add-ip {
-  width: auto;
+  width: 100px;
   height: 40px;
   padding: 0 16px;
   background-color: #3ba4ed;
@@ -410,20 +411,23 @@
   cursor: pointer;
   transition: background-color 0.2s ease;
   flex-shrink: 0;
+  display: flex;
+  align-items: center;
+  justify-content: center;
 
   &:hover {
     background-color: #2b94dd;
   }
 
   @include respond-to('md') {
-    width: auto;
+    width: 100%;
     font-size: 14px;
     height: 38px;
   }
 
   // Figma 모바일 (1290-3137): 89px × 40px, border-radius 8px, 12px bold
   @include respond-to('sm') {
-    width: auto;
+    width: 89px;
     height: 36px;
     font-size: 13px;
     border-radius: 8px;
@@ -446,6 +450,7 @@
 
   .ip-item {
     display: flex;
+    align-items: center;
     gap: 21px;
     padding: 0;
     border-bottom: none;
@@ -488,25 +493,26 @@
     }
 
     .btn-remove-ip {
-      width: 158px !important;
-      height: 60px !important;
+      width: 100px !important;
+      height: 40px !important;
       background-color: #e5e7eb !important;
       color: #5f666c !important;
       border: none !important;
-      border-radius: $border-radius-lg !important;
-      font-size: 20px !important;
-      font-weight: $font-weight-bold;
+      border-radius: $border-radius-md !important;
+      font-size: 14px !important;
+      font-weight: $font-weight-semibold;
       cursor: pointer;
       transition: background-color 0.2s ease;
       flex-shrink: 0;
       display: flex !important;
       align-items: center;
       justify-content: center;
+      align-self: center;
 
       @include respond-to('md') {
         width: 100% !important;
-        font-size: 16px !important;
-        height: 50px !important;
+        font-size: 14px !important;
+        height: 38px !important;
       }
 
       // Figma 모바일 (1290-3137): 89px × 40px, border-radius 8px, 12px bold