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/package.json

@@ -7,6 +7,7 @@
     "@react-native-async-storage/async-storage": "2.2.0",
     "expo": "~57.0.11",
     "expo-constants": "~57.0.9",
+    "expo-crypto": "~57.0.1",
     "expo-device": "~57.0.1",
     "expo-font": "~57.0.1",
     "expo-glass-effect": "~57.0.1",