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/onboarding.ts

@@ -12,8 +12,10 @@
 //                                       suppresses the auto-popup on future logins (they
 //                                       can still reopen it from Settings → Account).
 
-export const ONBOARD_ACTIVE = "siftlode.onboarding.active";
-export const ONBOARD_DISMISSED = "siftlode.onboarding.dismissed";
+import { LS } from "./storage";
+
+export const ONBOARD_ACTIVE = "siftlode.onboarding.active"; // sessionStorage (not in LS, which is localStorage)
+export const ONBOARD_DISMISSED = LS.onboardingDismissed;
 
 export function shouldAutoOpenOnboarding(me: {
   can_read: boolean;