feat(library): update, rescan and stop on the folder menu, queued per roll

A folder row now asks for the rest of a roll, the whole of it from the top,
or takes its request back. Requests queue behind the reading in flight and
run one at a time; a row waiting its turn draws a ring that does not turn.

docker/frontend/src/styles/app.css

@@ -1161,6 +1161,9 @@ button.hint:disabled { cursor: default; }
   animation: lib-spin 0.7s linear infinite;
 }
 @keyframes lib-spin { to { transform: rotate(360deg); } }
+/* The ring of a reading that is queued: still, and greyed, so it reads as a
+   wait rather than as work. */
+.lib-spin.wait { animation: none; border-top-color: var(--border); opacity: 0.6; }
 /* The same ring, out in the header: the reading belongs to this tab, not to a
    screen, so the studio says so too while it runs. */
 .lib-scan { display: inline-flex; align-items: center; gap: 6px; white-space: nowrap; }