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/NavSidebar.tsx

@@ -23,6 +23,7 @@ import {
 import { api, type Me } from "../lib/api";
 import { useLiveQuery } from "../lib/useLiveQuery";
 import { getUnreadCount, subscribe } from "../lib/notifications";
+import { LS } from "../lib/storage";
 import type { Page } from "../lib/urlState";
 import { type LangCode } from "../i18n";
 import AvatarImg from "./Avatar";
@@ -48,7 +49,7 @@ export default function NavSidebar({
 }) {
   const { t } = useTranslation();
   const [collapsed, setCollapsed] = useState<boolean>(
-    () => localStorage.getItem("siftlode.navCollapsed") === "1"
+    () => localStorage.getItem(LS.navCollapsed) === "1"
   );
   const [acctOpen, setAcctOpen] = useState(false);
   const acctBtnRef = useRef<HTMLButtonElement | null>(null);
@@ -90,7 +91,7 @@ export default function NavSidebar({
   function toggleCollapsed() {
     setCollapsed((c) => {
       const next = !c;
-      localStorage.setItem("siftlode.navCollapsed", next ? "1" : "0");
+      localStorage.setItem(LS.navCollapsed, next ? "1" : "0");
       return next;
     });
   }