Fix #4822: Add dismissible close button to announcement banner

[ronit-1404]

Description

This PR adds a close button to the announcement banner, allowing users to dismiss it when desired.

Changes Made

• Added a close button (×) in the top-right corner of the announcement banner
• Implemented dismiss functionality that hides the banner on click
• Added CSS styling for the close button with hover effects
• Adjusted main header position when banner is dismissed

Files Modified

• frontend/src/views/web/partials/main-header.html - Added close button element with onclick handler
• frontend/src/css/modules/main-header.scss - Added styling for the close button

Related Issue

Fixes #4822

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.90%. Comparing base (d4ce4fa) to head (3184dc6).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4823      +/-   ##
==========================================
+ Coverage   91.89%   91.90%   +0.01%     
==========================================
  Files          86       86              
  Lines        7132     7132              
==========================================
+ Hits         6554     6555       +1     
+ Misses        578      577       -1     

Flag	Coverage Δ
backend	96.67% <ø> (ø)
frontend	87.17% <ø> (+0.02%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
Accounts & Authentication	100.00% <ø> (ø)
Challenges Management	95.39% <ø> (ø)
Job Processing	98.19% <ø> (ø)
Participants & Teams	100.00% <ø> (ø)
Challenge Hosts	100.00% <ø> (ø)
Analytics	100.00% <ø> (ø)
Web Interface	100.00% <ø> (ø)
Frontend (Gulp)	87.17% <ø> (+0.02%)	⬆️
All Models	97.58% <ø> (ø)
All Views	100.00% <ø> (ø)
All Serializers	98.82% <ø> (ø)
Utility Functions	96.82% <ø> (ø)
Core Configuration	82.35% <ø> (ø)
see 1 file with indirect coverage changes
see 1 file with indirect coverage changes

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