✨ feat: remove image upload limit and improve scrollbar styling

Remove the 5-image upload restriction in playground and enhance UI consistency

Changes:
- Remove 5-image limit constraint from ImageUrlInput component
- Update hint text to remove "maximum 5 images" references
- Add custom scrollbar styling for image list to match site-wide design
- Apply consistent thin scrollbar (6px width) with Semi Design color variables
- Maintain hover effects and rounded corners for better UX

Breaking Changes: None

Files modified:
- web/src/components/playground/ImageUrlInput.js
- web/src/index.css

This change allows users to upload unlimited images in playground mode while
maintaining visual consistency across the application's scrollable elements.

web/src/index.css

@@ -236,6 +236,25 @@ code {
   scrollbar-width: none;
 }
 
+/* 图片列表滚动条样式 */
+.image-list-scroll::-webkit-scrollbar {
+  width: 6px;
+  height: 6px;
+}
+
+.image-list-scroll::-webkit-scrollbar-thumb {
+  background: var(--semi-color-tertiary-light-default);
+  border-radius: 3px;
+}
+
+.image-list-scroll::-webkit-scrollbar-thumb:hover {
+  background: var(--semi-color-tertiary);
+}
+
+.image-list-scroll::-webkit-scrollbar-track {
+  background: transparent;
+}
+
 /* 隐藏请求体 JSON TextArea 的滚动条 */
 .custom-request-textarea .semi-input::-webkit-scrollbar {
   display: none;