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/notifications.ts

@@ -2,6 +2,7 @@
 // the Notification Center shows (info events, actions awaiting interaction, and app
 // errors). History is persisted to localStorage so it survives reloads; action
 // callbacks are live-only and dropped on reload.
+import { LS, readJSON, readMerged, writeJSON } from "./storage";
 
 export type NotifLevel = "info" | "success" | "warning" | "error" | "fatal";
 
@@ -71,8 +72,8 @@ export interface NotifyInput {
   transient?: boolean; // live status: sticky toast, not persisted (see Notification.transient)
 }
 
-const HISTORY_KEY = "siftlode.notifications";
-const SETTINGS_KEY = "siftlode.notifSettings";
+const HISTORY_KEY = LS.notifHistory;
+const SETTINGS_KEY = LS.notifSettings;
 const MAX_HISTORY = 100;
 const ERROR_TTL = 15000;
 
@@ -86,11 +87,7 @@ const DEFAULT_SETTINGS: NotifSettings = { sound: false, durationMs: 6000 };
 let config: NotifSettings = loadSettings();
 
 function loadSettings(): NotifSettings {
-  try {
-    return { ...DEFAULT_SETTINGS, ...JSON.parse(localStorage.getItem(SETTINGS_KEY) || "{}") };
-  } catch {
-    return DEFAULT_SETTINGS;
-  }
+  return readMerged(SETTINGS_KEY, DEFAULT_SETTINGS);
 }
 
 export function getNotifSettings(): NotifSettings {
@@ -99,11 +96,7 @@ export function getNotifSettings(): NotifSettings {
 
 export function configureNotifications(p: Partial<NotifSettings>): void {
   config = { ...config, ...p };
-  try {
-    localStorage.setItem(SETTINGS_KEY, JSON.stringify(config));
-  } catch {
-    /* ignore */
-  }
+  writeJSON(SETTINGS_KEY, config);
 }
 
 let audioCtx: AudioContext | null = null;
@@ -137,7 +130,7 @@ let cachedUnread = 0;
 
 function load(): Notification[] {
   try {
-    const raw = JSON.parse(localStorage.getItem(HISTORY_KEY) || "[]");
+    const raw = readJSON<unknown>(HISTORY_KEY, []);
     if (!Array.isArray(raw)) return [];
     // Restored entries are history only. Mark them dismissed so they don't
     // resurrect as active toasts on reload — their auto-dismiss timers aren't
@@ -154,7 +147,7 @@ function persist() {
     // Transient status notices are live-session only — never write them to history,
     // so a reload can't leave one stranded with no producer left to clear it.
     const slim = items.filter((n) => !n.transient).map(({ action: _action, ...n }) => n);
-    localStorage.setItem(HISTORY_KEY, JSON.stringify(slim));
+    writeJSON(HISTORY_KEY, slim);
   } catch {
     /* ignore quota / serialization errors */
   }