style: improved visual separation and aesthetic

- Added a border radius to scrollbar thumb styles for improved aesthetic.
- Updated the Divider component to include a border for better visual separation.
- Added border to the divider in SelectModelPopup for improved visibility.

src/renderer/src/assets/styles/scrollbar.scss

@@ -10,6 +10,7 @@
 
 ::-webkit-scrollbar-thumb {
   background: var(--color-scrollbar-thumb);
+  border-radius: 4px;
   &:hover {
     background: var(--color-scrollbar-thumb-hover);
   }

src/renderer/src/components/Popups/AddAssistantPopup.tsx

@@ -102,7 +102,7 @@ const PopupContainer: React.FC<Props> = ({ resolve }) => {
           size="middle"
         />
       </HStack>
-      <Divider style={{ margin: 0 }} />
+      <Divider style={{ margin: 0, borderBlockStartWidth: 0.5 }} />
       <Container>
         {agents.map((agent) => (
           <AgentItem

src/renderer/src/components/Popups/SelectModelPopup.tsx

@@ -100,7 +100,7 @@ const PopupContainer: React.FC<PopupContainerProps> = ({ model, resolve }) => {
           size="middle"
         />
       </HStack>
-      <Divider style={{ margin: 0 }} />
+      <Divider style={{ margin: 0, borderBlockStartWidth: 0.5 }} />
       <Container>
         {filteredItems.length > 0 ? (
           <StyledMenu
@@ -123,9 +123,6 @@ const Container = styled.div`
   height: 50vh;
   margin-top: 10px;
   overflow-y: auto;
-  &::-webkit-scrollbar {
-    display: none;
-  }
 `
 
 const StyledMenu = styled(Menu)`