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>

CHANGELOG.md

@@ -20,6 +20,10 @@ All notable changes to LLeMbas Desktop. The format follows
   server says it, unless this computer's or another of the app's nine is chosen (kept in the app;
   the window opens again in it). The theme is the account's, and choosing one saves it there, so
   the web interface and the phone follow; on a server that cannot be told, it is kept in the app.
+- **Pinned chats and chat folders.** The sidebar lists the pinned chats first, then the account's
+  folders, nested and folded as the server keeps them, then the rest by day. A chat's menu pins
+  it and moves it into a folder or a new one; a folder's menu renames it, makes one inside it, or
+  deletes it (its chats stay, back in the list). The search line still lists every match.
 
 ### Fixed