Add close button to announcement banner

## Description
The announcement banner on the homepage is fixed at the top of the page and remains visible even when scrolling. Currently, there is no way for users to dismiss this banner, which can be intrusive for users who have already read the announcement.

## Proposed Solution
Add a close button (×) to the top-right corner of the announcement banner that allows users to dismiss it.

## Expected Behavior
- A close button should be visible in the top-right corner of the announcement banner
- Clicking the close button should hide the banner
- The main navigation should adjust to the top position when the banner is dismissed
- The close button should have hover effects for better UX

## Current Behavior
The announcement banner is permanently fixed at the top with no option to close it.

## Screenshots

Start by locating the homepage announcement banner and main navigation implementation in the frontend. Check how the banner is positioned and how navigation spacing is calculated. Done means a top-right close button dismisses the banner, navigation moves to the top, and the button has hover feedback.

Written by the indexing model from the issue text.