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/components/Tabs.tsx

@@ -1,4 +1,4 @@
-import { useState } from "react";
+import { usePersistedState } from "../lib/storage";
 
 // Reusable horizontal pill tab-rail for in-page sub-navigation (Configuration, Users & roles…).
 // The active tab is persisted to localStorage so a reload (F5) keeps the user where they were,
@@ -10,16 +10,9 @@ export interface TabDef {
   badge?: number; // optional count pill (e.g. pending requests); hidden when 0/undefined
 }
 
-/** Persisted active-tab state. Validation is deferred to the caller (it clamps to a valid id at
- * render) so this can be called before an async-loaded tab list is known, keeping hook order stable. */
-export function usePersistedTab(storageKey: string, fallback = ""): [string, (id: string) => void] {
-  const [tab, setTabState] = useState<string>(() => localStorage.getItem(storageKey) ?? fallback);
-  const setTab = (id: string) => {
-    setTabState(id);
-    localStorage.setItem(storageKey, id);
-  };
-  return [tab, setTab];
-}
+/** Persisted active-tab state — the canonical helper now lives in lib/storage as
+ * `usePersistedState`; re-exported here under its original name for existing call sites. */
+export const usePersistedTab = usePersistedState;
 
 export default function Tabs({
   tabs,