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

tasks/ch21-30.md

@@ -145,14 +145,14 @@
 
 ## T23d AI Chat · Conversations
 
-- [ ] ConversationList
-- [ ] ConversationItem
-- [ ] ConversationSearch
-- [ ] NewChatButton
-- [ ] FolderList / ProjectList
-- [ ] ProjectKnowledgePanel
-- [ ] SharedConversationView
-- [ ] ConversationExport
+- [x] ConversationList
+- [x] ConversationItem
+- [x] ConversationSearch → `forms::SearchInput` over `chat::ConversationList::query`
+- [x] NewChatButton → `chat::new_chat_button`, a `buttons::Button`
+- [x] FolderList / ProjectList → `chat::ProjectList`
+- [x] ProjectKnowledgePanel
+- [x] SharedConversationView
+- [x] ConversationExport
 
 ## T23e AI Chat · Welcome