feat(frontend): add createStore factory + central localStorage registry

- lib/store.ts: createStore<T> — one observable-value primitive (get/set/subscribe/use)
  replacing the hand-rolled listeners/subscribe/emit triad several modules each had.
- lib/storage.ts: LS key registry (every siftlode.* key in one place), readMerged/
  readJSON/writeJSON (the try/JSON.parse/merge/catch done once), and usePersistedState
  (the reactive persisted-string hook, generalizing usePersistedTab).

frontend/src/lib/storage.ts

@@ -0,0 +1,80 @@
+import { useState } from "react";
+
+// Central registry of every localStorage key the app uses. One place to see them all, rename
+// safely, and avoid collisions — instead of `"siftlode.x"` string literals scattered across
+// files. Parametric (per-user) keys are functions. All keys are namespaced `siftlode.*`.
+export const LS = {
+  theme: "siftlode.theme",
+  sidebarLayout: "siftlode.sidebarLayout",
+  hints: "siftlode.hints",
+  lang: "siftlode.lang",
+  filters: "siftlode.filters",
+  page: "siftlode.page",
+  perfMode: "siftlode.perfMode",
+  channelFilter: "siftlode.channelFilter",
+  channelsView: "siftlode.channelsView",
+  settingsTab: "siftlode.settingsTab",
+  statsTab: "siftlode.statsTab",
+  adminUsersTab: "siftlode.adminUsersTab",
+  navCollapsed: "siftlode.navCollapsed",
+  playlist: "siftlode.playlist",
+  plSort: "siftlode.plSort",
+  notifHistory: "siftlode.notifications",
+  notifSettings: "siftlode.notifSettings",
+  onboardingDismissed: "siftlode.onboarding.dismissed",
+  seenVersion: "siftlode.seenVersion",
+  chatDock: (userId: number) => `siftlode.chatDock.${userId}`,
+} as const;
+
+// --- typed read/write helpers (do the JSON + try/catch once) ----------------------------
+
+/** Read an object-shaped value merged over `defaults` (so missing/added keys are tolerated),
+ * falling back to `defaults` on missing or corrupt data. */
+export function readMerged<T extends object>(key: string, defaults: T): T {
+  try {
+    return { ...defaults, ...JSON.parse(localStorage.getItem(key) || "{}") };
+  } catch {
+    return { ...defaults };
+  }
+}
+
+/** Read any JSON value (arrays/scalars), falling back on missing or corrupt data. */
+export function readJSON<T>(key: string, fallback: T): T {
+  try {
+    const raw = localStorage.getItem(key);
+    return raw == null ? fallback : (JSON.parse(raw) as T);
+  } catch {
+    return fallback;
+  }
+}
+
+/** Persist a value as JSON, swallowing quota/serialization errors. */
+export function writeJSON(key: string, value: unknown): void {
+  try {
+    localStorage.setItem(key, JSON.stringify(value));
+  } catch {
+    /* ignore quota / serialization errors */
+  }
+}
+
+// --- reactive persisted string state ----------------------------------------------------
+
+/** A `useState` whose string value is mirrored to localStorage, so it survives a reload (F5).
+ * Validation is deferred to the caller (clamp at render) so it can be used before an
+ * async-loaded option list is known, keeping hook order stable. Generalizes the per-component
+ * "persisted active tab" pattern (Settings, Stats, admin pages, the channel filter…). */
+export function usePersistedState(
+  key: string,
+  fallback = "",
+): [string, (value: string) => void] {
+  const [value, setValue] = useState<string>(() => localStorage.getItem(key) ?? fallback);
+  const set = (next: string) => {
+    setValue(next);
+    try {
+      localStorage.setItem(key, next);
+    } catch {
+      /* ignore */
+    }
+  };
+  return [value, set];
+}