feat: implement PWA offline support with pending transaction queue

[Davichi-1]

Overview

This PR adds full PWA offline support to split-app with service worker caching and an IndexedDB-backed pending transaction queue. Invoice reads from the last session are served from cache when offline. Pending pay/approve transactions submitted while offline are queued in IndexedDB and auto-submitted when connectivity is restored. An offline banner communicates queue status, and the manifest uses correct StellarSplit branding.

Related Issue

Closes #

Changes

📱 PWA & Service Worker

• [MODIFY] next.config.js

  • Configure next-pwa for service worker generation.
  • Cache strategy: network-first for API routes, cache-first for static assets.

• [ADD] public/sw.js

  • Runtime caching for the app shell, static assets, and API responses.
  • Offline fallback for invoice data from the last session.

• [MODIFY] public/manifest.json

  • StellarSplit branding: name, icons, start_url, display standalone, and theme color #0B1628.

• [MODIFY] src/app/layout.tsx

  • Add manifest link and theme-color metadata for PWA installability on mobile and desktop.

• [MODIFY] package.json

  • Add next-pwa, idb, and test-related dependencies/scripts.

🔌 Offline Queue & IndexedDB

• [ADD] src/lib/offlineQueue.ts

  • IndexedDB-backed pending transaction queue using idb.
  • When a pay/approve transaction fails due to a network error, store it in the queue.
  • On reconnect, drain the queue automatically with retry logic; remove items only after successful submission.

• [ADD] src/components/OfflineQueueSync.tsx

  • Listens for online/offline events and triggers queue drain.
  • Shows the offline banner: "You're offline — pending transactions will submit when you reconnect".

🧪 Tests

• [ADD] src/lib/__tests__/offlineQueue.test.ts
  • Unit tests for queue drain logic: ordering, retry on failure, clearing only successful transactions, and reconnect-triggered draining.

Verification Results

npm run lint
✅ 0 errors

npx tsc --noEmit
✅ 0 errors

npm run build
✅ Build succeeded

npm test -- src/lib/__tests__/offlineQueue.test.ts
✅ 8/8 passed

Manual PWA checks:
✅ Installable on mobile/desktop via manifest + service worker
✅ Invoice data visible offline from cache
✅ Pending transaction queued offline and auto-submitted on reconnect
✅ Offline banner rendered correctly

Acceptance Criteria	Status
App installs as PWA on mobile and desktop	✅ Manifest + service worker registered; installable build verified
Invoice data visible offline from cache	✅ Network-first API with cache-first static assets; last session invoice data cached
Pending transactions queue and auto-submit on reconnect	✅ IndexedDB queue drains on online event with retry logic
Offline banner shown correctly	✅ OfflineQueueSync renders banner on connection loss
manifest.json has correct StellarSplit branding	✅ Name, icons, standalone display, theme #0B1628
npm run lint passes with zero errors	✅ 0 errors
TypeScript compiles with zero errors	✅ 0 errors
npm run build succeeds	✅ Build succeeded

Closes #385

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