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>

ui/app.css

@@ -1629,6 +1629,40 @@ details[open] > summary .step__chev .icon { transform: rotate(90deg); }
 .findings__item { min-width: 0; overflow-wrap: anywhere; }
 .ask__subagent { color: var(--ink); }
 
+/* --- Archived chats: icon · title over its folder and date · Unarchive · more, on known tracks. */
+
+.arch { list-style: none; margin: 0; padding: 0; display: grid; }
+.arow {
+  display: grid;
+  grid-template-columns: 16px minmax(0, 1fr) auto var(--control-h-sm);
+  gap: var(--sp-3);
+  align-items: center;
+  min-height: var(--control-h);
+  padding: var(--sp-1) 0;
+  border-bottom: var(--border-w) solid var(--border);
+}
+.arow__icon { display: grid; place-items: center; color: var(--ink-faint); }
+.arow__icon.is-agent { color: var(--leaf); }
+.arow__icon .icon { width: 14px; height: 14px; }
+.arow__open {
+  display: grid;
+  min-width: 0;
+  min-height: var(--control-h);
+  padding: var(--sp-1) 0;
+  text-align: left;
+  color: var(--ink);
+  background: transparent;
+  border: 0;
+  cursor: pointer;
+}
+.arow__open:hover .arow__title { text-decoration: underline; }
+.arow__title, .arow__sub { overflow: hidden; white-space: nowrap; text-overflow: ellipsis; }
+.arow__title { font: var(--text-sm) / var(--leading-tight) var(--font-body); }
+.arow__sub { font: var(--text-xs) var(--font-ui); color: var(--ink-faint); }
+.arch__more { margin-top: var(--sp-3); }
+.arch__page { display: grid; gap: var(--sp-3); }
+.head__unarchive { white-space: nowrap; }
+
 /* --- Narrower windows ---------------------------------------------------------------------- */
 
 /* 64rem (--bp-wide): a panel goes over the chat instead of beside it. */