refactor(frontend): adopt store/storage helpers; centralize persistence

- errorDialog + hints now use createStore (drop their bespoke listener arrays).
- theme/sidebarLayout/notifications/App filters/Playlists plSort use readMerged/
  readJSON/writeJSON instead of inline try/JSON.parse/catch.
- Stats, SettingsPanel and App's channel filter/view tabs use usePersistedState
  (the 3 sites that reinvented usePersistedTab inline).
- Every siftlode.* key now sourced from the LS registry (no scattered literals).

frontend/src/components/Playlists.tsx

@@ -37,6 +37,7 @@ import {
 import { api, type Playlist, type Video } from "../lib/api";
 import { formatDuration } from "../lib/format";
 import { notify } from "../lib/notifications";
+import { LS, readMerged, writeJSON } from "../lib/storage";
 import { useUndoable } from "../lib/useUndoable";
 import PlayerModal from "./PlayerModal";
 import UndoToolbar from "./UndoToolbar";
@@ -187,26 +188,18 @@ export default function Playlists({ canWrite }: { canWrite: boolean }) {
   const confirm = useConfirm();
   // Persist the selected playlist so F5 keeps it (the de-URL refactor dropped it from the URL).
   const [selectedId, setSelectedId] = useState<number | null>(() => {
-    const s = localStorage.getItem("siftlode.playlist");
+    const s = localStorage.getItem(LS.playlist);
     return s ? Number(s) : null;
   });
   useEffect(() => {
-    if (selectedId != null) localStorage.setItem("siftlode.playlist", String(selectedId));
+    if (selectedId != null) localStorage.setItem(LS.playlist, String(selectedId));
   }, [selectedId]);
   const selectedRef = useRef<HTMLButtonElement | null>(null);
   const scrolledRef = useRef(false);
   // How the left playlist rail is ordered (persisted).
-  const [plSort, setPlSort] = useState<PlSort>(() => {
-    try {
-      const s = localStorage.getItem("siftlode.plSort");
-      if (s) return { ...PL_SORT_DEFAULT, ...JSON.parse(s) };
-    } catch {
-      /* ignore */
-    }
-    return PL_SORT_DEFAULT;
-  });
+  const [plSort, setPlSort] = useState<PlSort>(() => readMerged(LS.plSort, PL_SORT_DEFAULT));
   useEffect(() => {
-    localStorage.setItem("siftlode.plSort", JSON.stringify(plSort));
+    writeJSON(LS.plSort, plSort);
   }, [plSort]);
   const [newName, setNewName] = useState("");
   const [renaming, setRenaming] = useState(false);