Archived chats: a page to find them again, Restore chat, delete

Archive was one-way in the app. Archived (sidebar, under Files) reads
GET /api/v2/chats?archived=true, newest first and paged; a row opens the
chat, Restore chat PATCHes archived:false and reads the sidebar again, and
the row's menu deletes. An open archived chat says so in its header, with
Restore chat there and in its menu. The words are the web's own ("Archived",
"Restore chat"). Tests: D2 at 1280 and 390 px.

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

CHANGELOG.md

@@ -4,6 +4,15 @@ All notable changes to LLeMbas Desktop. The format follows
 [Keep a Changelog](https://keepachangelog.com/en/1.1.0/), and the project uses
 [Semantic Versioning](https://semver.org/).
 
+## [Unreleased]
+
+### Added
+
+- **Archived chats.** Archive chat was one-way: an archived chat left the sidebar and could not be
+  found again in the app. Archived, under Files in the sidebar, lists them newest first; each opens
+  as any chat does, goes back into the sidebar with Restore chat, or is deleted. An archived chat
+  that is open says so in its header, with Restore chat beside it and in its menu.
+
 ## [1.0.0-beta.6] — 2026-11-10
 
 LLeMbas CLI 2.0.0-beta.6 inside, the look and words of LLeMbas 2.0.0-beta.8.