Core implement offline first with queue and sync

[OG-wura]

Close #12

PR description:
Summary
Implements offline support for the StepFi app. When the device loses connectivity, mutation requests (repay, deposit, vouch, create loan, submit signed XDR) are queued to persisted storage and automatically replayed when connectivity is restored. GET responses are cached with a 5-minute TTL and served from cache on network failure to allow browsing previously loaded data offline.
Changes
New modules (src/offline/)

• cache.ts — AsyncStorage-backed response cache with configurable TTL (default 5 min). Stores { data, timestamp, ttl } entries keyed by GET:.
• offline-queue.ts — Persisted FIFO action queue. Supports enqueueAction, dequeueAction, getQueue, clearQueue. Each action stores { id, type, endpoint, method, data, timestamp }.
• offline-sync.ts — Queue processor. Iterates over queued actions sequentially via api.request(), removes successfully replayed ones, emits sync:start/complete/error/progress events for UI feedback.
• connectivity.store.ts — Zustand store tracking isConnected, connectionType, isInternetReachable, updated by the NetInfo listener in the root layout.
  New hook
• hooks/useConnectivity.ts — React hook exposing { isConnected, connectionType, isInternetReachable } from the connectivity store.
  Modified files
• services/api.ts — Three interceptor additions:

1. Request interceptor (mutations): Checks connectivity.store; if offline, queues the action via enqueueAction and rejects the request with __offline_queued.
2. Response success interceptor: Caches all successful GET responses.
3. Response error interceptor: Returns mock 202 for offline-queued mutations (avoids error in caller); serves cached data for failed GET requests; existing 401/refresh flow unchanged.

• app/_layout.tsx — On mount, fetches initial NetInfo state and subscribes to connectivity changes. Updates the connectivity store on every change. When transitioning from offline → online, triggers processQueue() to drain the action queue.
  Dependencies added
• @react-native-community/netinfo
• @react-native-async-storage/async-storage
  Verification
• TypeScript: no new errors (npx tsc --noEmit)
• Lint: no new errors (npx eslint)
• All pre-existing errors (Node v18 Metro compat, CalendarSourceType) are unrelated