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/hints.ts

@@ -1,36 +1,27 @@
 // App-wide "hints" toggle: when on, Tooltip components reveal short explanatory
 // captions on hover so the UI is somewhat self-documenting for new users. Experienced
 // users can turn it off in Settings. Persisted to localStorage (+ server preferences).
-const KEY = "siftlode.hints";
-
-let enabled = load();
-let listeners: Array<() => void> = [];
+import { createStore } from "./store";
+import { LS } from "./storage";
 
 function load(): boolean {
   try {
-    return localStorage.getItem(KEY) !== "0"; // default ON
+    return localStorage.getItem(LS.hints) !== "0"; // default ON
   } catch {
     return true;
   }
 }
 
-export function hintsEnabled(): boolean {
-  return enabled;
-}
+const store = createStore<boolean>(load());
+
+export const hintsEnabled = store.get;
+export const subscribeHints = store.subscribe;
 
 export function setHintsEnabled(v: boolean): void {
-  enabled = v;
+  store.set(v);
   try {
-    localStorage.setItem(KEY, v ? "1" : "0");
+    localStorage.setItem(LS.hints, v ? "1" : "0");
   } catch {
     /* ignore */
   }
-  listeners.forEach((l) => l());
-}
-
-export function subscribeHints(listener: () => void): () => void {
-  listeners.push(listener);
-  return () => {
-    listeners = listeners.filter((l) => l !== listener);
-  };
 }