Horizons chat UX improvements

Two new UX features are now available in Horizons chat. You can select any text in the chat and hit Reply to attach it to your next message, so the AI understands exactly what you're referring to without needing to copy-paste or describe the reference. On desktop, the chat panel is now resizable by dragging its edge; your preferred width is saved across reloads, and you can drag it all the way to hide it or double-click the handle to reset to the default size.