Make the persistent pinned-chat icon clickable to unpin

[davidtaylor6130]

Problem

Pinned chats keep an always-visible pin indicator, but that indicator is passive. A user cannot click the most obvious pinned-state control to unpin the chat and must find the separate action instead.

This follows up on #150, which deliberately retained a passive marker for pinned chats.

Expected behaviour

Make the always-visible pin indicator on a pinned chat an interactive unpin button.

Acceptance criteria

• A pinned chat continues to show its pin control without requiring hover.
• Clicking the visible pin unpins that chat.
• The control is keyboard accessible and has an explicit accessible label such as Unpin <chat name>.
• Activating the pin does not also select/open the chat or begin a drag operation.
• Existing pinning, row hover actions, layout, and unpinned-chat behaviour remain unchanged.
• Add or update focused UI tests for mouse and keyboard unpinning from the persistent pin control.