💄 style: hide announcement modal scrollbar

Improve UX by hiding the vertical scrollbar inside the announcement (NoticeModal)
while keeping the content scrollable.

Changes
• NoticeModal.js
  - Introduce `notice-content-scroll` class to the content wrapper.
  - Remove inline custom scrollbar styling for cleaner code.

• index.css
  - Add `.notice-content-scroll` to the global hidden-scrollbar rules, ensuring
    scrollbars are hidden across browsers.

Result
Users can still scroll through long announcements, but no scrollbar is shown,
giving the modal a cleaner and more consistent appearance.

web/src/components/layout/NoticeModal.js

@@ -64,11 +64,7 @@ const NoticeModal = ({ visible, onClose, isMobile }) => {
     return (
       <div
         dangerouslySetInnerHTML={{ __html: noticeContent }}
-        className="max-h-[60vh] overflow-y-auto pr-2"
-        style={{
-          scrollbarWidth: 'thin',
-          scrollbarColor: 'var(--semi-color-tertiary) transparent'
-        }}
+        className="notice-content-scroll max-h-[60vh] overflow-y-auto pr-2"
       />
     );
   };