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/theme.ts

@@ -1,3 +1,5 @@
+import { LS, readMerged, writeJSON } from "./storage";
+
 export type Mode = "dark" | "light";
 export type Scheme = "midnight" | "forest" | "slate" | "youtube";
 
@@ -27,16 +29,10 @@ export function applyTheme(t: ThemePrefs): void {
   el.style.setProperty("--font-scale", String(t.fontScale));
 }
 
-const KEY = "siftlode.theme";
-
 export function loadLocalTheme(): ThemePrefs {
-  try {
-    return { ...DEFAULT_THEME, ...JSON.parse(localStorage.getItem(KEY) || "{}") };
-  } catch {
-    return DEFAULT_THEME;
-  }
+  return readMerged(LS.theme, DEFAULT_THEME);
 }
 
 export function saveLocalTheme(t: ThemePrefs): void {
-  localStorage.setItem(KEY, JSON.stringify(t));
+  writeJSON(LS.theme, t);
 }