Add a configurable warning/info banner

[providenz]

Feature Request

When we need to inform users about planned maintenance, service degradation, new features, or any important announcement, we currently have no way to display a temporary message in the assistant application.

Describe the solution you'd like

Add a configurable warning/info banner that can be managed directly from the Django admin interface. The banner should support:

• Enable/disable toggle: Show or hide the banner without deleting the content

• Message content: Rich text or markdown support for the announcement

• Severity level: Different styles for info, warning, or critical messages according to dsfr

• Dismissible option: Allow users to dismiss the banner (stored locally)

The banner would appear at the top of the application (above the main content) and persist until dismissed.

[eliott07]

Great idea. Where would this banner be configured ? Should there be an interface in the Django Admin ?
We'll need some design from @rl-83 on the long run but seems very important and relatively urgent for the experimentation phase