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

assets/icons/corner-down-right.svg

@@ -0,0 +1,16 @@
+<!-- @license lucide-static v1.48.0 - ISC -->
+<svg
+  class="lucide lucide-corner-down-right"
+  xmlns="http://www.w3.org/2000/svg"
+  width="24"
+  height="24"
+  viewBox="0 0 24 24"
+  fill="none"
+  stroke="currentColor"
+  stroke-width="2"
+  stroke-linecap="round"
+  stroke-linejoin="round"
+>
+  <path d="m15 10 5 5-5 5" />
+  <path d="M4 4v7a4 4 0 0 0 4 4h12" />
+</svg>