feat: add caching for backend objects

CLAUDE.md

@@ -55,6 +55,27 @@ You work with Patrick, a Solutions Architect, on the OYS Borrow a Boat app (oysq
 - Ionicons only (`ionicons/icons`) — no PrimeIcons
 - Always import individual icon names from `ionicons/icons` (tree-shakeable)
 
+### Offline Cache
+- **Rule**: Every table/view read from Supabase must be written to `useAppCache` on success and read from it when offline.
+- **Composables**: `useAppCache` (localStorage, 24h TTL), `useOfflineStatus` (reactive `isOnline`)
+- **Pattern** for any data fetch:
+  ```typescript
+  const cache = useAppCache()
+  const { isOnline } = useOfflineStatus()
+
+  if (!isOnline.value) {
+    const cached = cache.peek<T>('my-key')
+    if (cached) data.value = cached
+    return
+  }
+  const { data: fresh } = await supabase.from('my_table').select('*')
+  data.value = fresh ?? []
+  if (fresh) cache.set('my-key', fresh)
+  ```
+- **Schedule data** is keyed by ISO week Monday: `cache.weekKey(utcIso)` → use keys `intervals:{monday}` and `slots:{monday}`.
+- **Realtime**: `app/app.vue` subscribes to `reservations`, `intervals`, and `boats` changes and patches the cache in real time. When adding a new table subscription, add it to the `app-cache-sync` channel in `app.vue`.
+- **Cross-page navigation state** (not persistence): use `useBookingDraft` as a pattern — module-level `ref` set by the source page, consumed (`take()`) by the destination page. Do not use query params for structured objects.
+
 ## Rules
 
 1. Do not mix unrelated project contexts in one session.