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/primitives/icon.rs

@@ -109,6 +109,7 @@ icons! {
     Compass => "compass",
     Contrast => "contrast",
     Copy => "copy",
+    CornerDownRight => "corner-down-right",
     Cpu => "cpu",
     CreditCard => "credit-card",
     Crop => "crop",
@@ -249,6 +250,7 @@ icons! {
     SquareCheck => "square-check",
     Square => "square",
     SquareFunction => "square-function",
+    SquarePen => "square-pen",
     Star => "star",
     StepForward => "step-forward",
     Strikethrough => "strikethrough",