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/lib/sidebarLayout.ts

@@ -4,6 +4,8 @@
 
 // `show`, `sort` and `content` moved to the feed toolbar (above the cards); they are no
 // longer sidebar widgets. normalizeLayout drops them from any persisted layout automatically.
+import { LS, readJSON, writeJSON } from "./storage";
+
 export type WidgetId = "date" | "language" | "topic" | "tags";
 
 export const ALL_WIDGETS: WidgetId[] = ["date", "language", "topic", "tags"];
@@ -27,8 +29,6 @@ export const DEFAULT_LAYOUT: SidebarLayout = {
   hidden: {},
 };
 
-const KEY = "siftlode.sidebarLayout";
-
 // Tolerate stale/partial data: keep only known widgets and append any that are missing
 // (e.g. a widget added in a later version) so nothing silently disappears.
 export function normalizeLayout(raw: unknown): SidebarLayout {
@@ -45,13 +45,9 @@ export function normalizeLayout(raw: unknown): SidebarLayout {
 }
 
 export function loadLayout(): SidebarLayout {
-  try {
-    return normalizeLayout(JSON.parse(localStorage.getItem(KEY) || "{}"));
-  } catch {
-    return DEFAULT_LAYOUT;
-  }
+  return normalizeLayout(readJSON<unknown>(LS.sidebarLayout, {}));
 }
 
 export function saveLayoutLocal(l: SidebarLayout): void {
-  localStorage.setItem(KEY, JSON.stringify(l));
+  writeJSON(LS.sidebarLayout, l);
 }