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/i18n/index.ts

@@ -1,5 +1,6 @@
 import i18n from "i18next";
 import { initReactI18next } from "react-i18next";
+import { LS } from "../lib/storage";
 
 // Endonyms (shown as-is regardless of the active language — the convention for language pickers).
 export const LANGUAGES = [
@@ -9,7 +10,7 @@ export const LANGUAGES = [
 ] as const;
 export type LangCode = (typeof LANGUAGES)[number]["code"];
 const SUPPORTED = LANGUAGES.map((l) => l.code) as readonly string[];
-export const LANG_KEY = "siftlode.lang";
+export const LANG_KEY = LS.lang;
 
 // Auto-load every locale file (locales/<lang>/<area>.json) and merge each <area> under one
 // `translation` namespace, so components use t("area.key") and adding an area needs no wiring.