Pinned chats and chat folders in the sidebar

The server's /api/v2 sets `pinned` on PATCH /chats/{id} and makes, renames and deletes
folders. The sidebar lists the pinned chats first, then the folders (nested, folded as the
server keeps them, each counting its chats), then the rest by day; searching lists every
match flat. A chat's menu pins it and moves it (to no folder, any folder, or a new one);
a folder's menu renames it, makes one inside it, deletes it (its chats stay). Tests F8.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

ui/js/main.js

@@ -267,6 +267,10 @@ async function boot() {
       showChat(id)
     },
     menu: (anchor, c) => chatMenu(anchor, c),
+    note: (text) => {
+      $("thread-note").textContent = text
+      $("thread-note").hidden = !text
+    },
   })
   initChat({
     status: () => status,