Local-first sync: on-device store + offline mutation queue, idempotent server upserts

Screens render synchronously from an AsyncStorage-backed store; every
mutation applies locally and drains to the API in the background
(last-write-wins, client-generated UUIDs). Sydney round-trips are off the
interaction path and the app works fully offline.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

app/src/lib/useStore.ts

@@ -0,0 +1,8 @@
+import { useSyncExternalStore } from "react";
+import { getState, subscribe, type State } from "./store";
+
+// Subscribe a component to the local-first store. The whole state object is
+// the snapshot (replaced immutably on every change), so derive lists inline.
+export function useStore(): State {
+  return useSyncExternalStore(subscribe, getState, getState);
+}