T23d: AI chat, conversations

- ConversationList: pinned first, then by day, newest first; a query keeps
  the titles that hold every word
- ConversationItem: a Tab stop when there is something to do; its menu of
  rename, pin and delete shows on hover, while open, or while the row holds
  focus; a press on the menu leaves the row shut
- new_chat_button, ProjectList, ProjectKnowledgePanel with a capacity meter
- SharedConversationView: who shared it and when, and a way to carry it on
- ConversationExport: format and thinking in a Dialog, Cancel or Export
- icons: square-pen, corner-down-right
- gallery: conversations with rename, pin and delete; projects; a shared
  conversation; export with a toast

src/chat/tests/mod.rs

@@ -9,6 +9,7 @@ use gpui::{
 
 mod cite;
 mod compose;
+mod history;
 
 use super::{
     ImageMessage, MessageAvatar, MessageBubble, MessageEditor, MessageList, Role, ThinkingBlock,