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/store.ts

@@ -0,0 +1,33 @@
+import { useSyncExternalStore } from "react";
+
+// A minimal observable value shared across React components AND non-React code (the api
+// layer, websocket handlers, etc.). Replaces the hand-rolled `listeners`/`subscribe`/`emit`
+// triad that several modules each re-implemented slightly differently.
+export interface Store<T> {
+  get(): T;
+  set(next: T | ((prev: T) => T)): void;
+  subscribe(listener: () => void): () => void;
+  /** Subscribe from a component; re-renders on every change. */
+  use(): T;
+}
+
+export function createStore<T>(initial: T): Store<T> {
+  let value = initial;
+  const listeners = new Set<() => void>();
+  const get = () => value;
+  const subscribe = (l: () => void) => {
+    listeners.add(l);
+    return () => {
+      listeners.delete(l);
+    };
+  };
+  return {
+    get,
+    set(next) {
+      value = typeof next === "function" ? (next as (p: T) => T)(value) : next;
+      listeners.forEach((l) => l());
+    },
+    subscribe,
+    use: () => useSyncExternalStore(subscribe, get, get),
+  };
+}