Add dismiss button to announcement banners

Description

The \"Claude Fable 5 is currently unavailable. Learn more\" banner sits directly above the input bar and cannot be dismissed. There is no close/X button or any way to hide it.

This banner appears even when Fable 5 is not selected as the active model — it's a global announcement shown to all users regardless of their configuration.

Problem

• The banner is persistent and takes up vertical space above the input bar
• There is no way to dismiss or close it
• It is not actionable for users who never used Fable 5 and are on a different model (e.g., Opus 4.6)
• Clicking "Learn more" does not dismiss it

Request

Add a close/dismiss button (e.g., an X) to announcement banners so users can acknowledge and hide them. Once dismissed, the banner should not reappear for the same announcement.

Environment

• Claude Code Desktop (Windows 11)
• Active model: Opus 4.6
• Banner: "Claude Fable 5 is currently unavailable. Learn more"

!banner screenshot — sits directly above input bar with no dismiss option