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/errorDialog.ts

@@ -1,4 +1,5 @@
 import i18n from "../i18n";
+import { createStore } from "./store";
 
 // A single, self-explanatory error dialog (modal) the user must acknowledge — used when the
 // server can't carry out an action (a definitive 4xx/5xx response). Distinct from toasts
@@ -9,30 +10,18 @@ export interface AppError {
   message: string;
 }
 
-let current: AppError | null = null;
-let listeners: Array<() => void> = [];
-const emit = () => listeners.forEach((l) => l());
+const store = createStore<AppError | null>(null);
 
 export function reportError(message?: string, title?: string): void {
-  current = {
+  store.set({
     title: title || i18n.t("errors.title"),
     message: message || i18n.t("errors.generic"),
-  };
-  emit();
+  });
 }
 
 export function dismissError(): void {
-  current = null;
-  emit();
+  store.set(null);
 }
 
-export function subscribeError(listener: () => void): () => void {
-  listeners.push(listener);
-  return () => {
-    listeners = listeners.filter((l) => l !== listener);
-  };
-}
-
-export function getError(): AppError | null {
-  return current;
-}
+export const subscribeError = store.subscribe;
+export const getError = store.get;