feat: responsive pause/stop, non-blocking update with progress bar, delete saved sessions (1.5.0)

- carve: check cancellation inside chunks and per hit; reassembly stage is cancellable
- status exposes `stopping`; UI shows "Pausing…" immediately
- Tauri update commands run off the UI thread; download percentage reported
- delete autosaved sessions manually (one/all) and by retention setting
- chore(release): 1.5.0

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

apps/gui/ui/app.css

@@ -542,3 +542,8 @@ dialog.lightbox[open] { display: grid; }
 .update-card .upd-actions { display: flex; gap: 8px; flex-wrap: wrap; }
 .update-card .sub { margin: 0; }
 pre.upd-notes { max-height: 50vh; overflow: auto; white-space: pre-wrap; font-size: 13px; }
+.update-card .upd-bar { margin-top: 10px; }
+
+.list li.has-del { position: relative; }
+.list li.has-del .del { position: absolute; right: 6px; top: 50%; transform: translateY(-50%); opacity: 0; }
+.list li.has-del:hover .del, .list li.has-del .del:focus-visible { opacity: 1; }