feat: allow setting centralised notification banner

[dcshzj]

Note

This is only a prototype to showcase to MOE for now

Problem

MOE need a notification banner that is centrally managed and immediately shown live on all MOE schools.

Solution

Breaking Changes

• Yes - this PR contains breaking changes
• No - this PR is backwards compatible

Features:

• Introduce a new CentralNotification component that pulls a specific JSON file from the S3 assets bucket and displays its content as a notification banner at the top of the site.
• Introduce a new /central-banner route on Isomer Studio to edit the central notification banner.

Screenshots

Screen.Recording.2026-02-23.at.20.57.24.mov

Tests

• Go to Isomer Studio and click on the Central Banner button at the top left of the screen.
• Verify that you are able to see the central notification banner settings page.
• Make sure that the staging sites are already built using this branch.
• Add a new central notification banner, verify that the banner immediately appears on the staging sites after a refresh.
• Make changes to the central notification banner and save, verify that the updated contents immediately appears on the staging sites after a refresh.

[mergify]

This pull request has been stale for more than 30 days! Could someone please take a look at it @opengovsg/isomer-engineers