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/version.ts

@@ -1,8 +1,10 @@
 // Build info baked into the SPA at build time (Vite inlines VITE_*-prefixed env).
 // Injected via Docker build-args (see Dockerfile); falls back to "dev" for un-stamped builds.
+import { LS } from "./storage";
+
 export const FRONTEND_VERSION = import.meta.env.VITE_APP_VERSION || "dev";
 export const FRONTEND_SHA = import.meta.env.VITE_GIT_SHA || "unknown";
 export const FRONTEND_BUILD_DATE = import.meta.env.VITE_BUILD_DATE || "";
 
 // Key for the "last version the user has seen release notes for" (drives the banner).
-export const SEEN_VERSION_KEY = "siftlode.seenVersion";
+export const SEEN_VERSION_KEY = LS.seenVersion;