feat: Add PWA support with offline dashboard and cached stream data

[Austinaminu2]

Description

As a financial app that users may check frequently on mobile, FlowStar would benefit from Progressive Web App (PWA) capabilities:

• Install to home screen on mobile
• Offline dashboard showing cached stream data with a "last updated" timestamp
• Background sync to refresh data when connectivity returns
• Push notifications for stream events (withdrawal available, stream completed)

Proposed Solution

Use next-pwa or a custom service worker:

1. Cache the app shell (HTML, CSS, JS) for offline access
2. Cache the last-fetched stream data in IndexedDB
3. Show cached data with a "You're offline — data may be outdated" banner
4. Add a manifest.json with FlowStar branding

Acceptance Criteria

• manifest.json with app name, icons, theme color, and display mode
• Service worker caches app shell for offline access
• Dashboard loads from cache when offline (with stale indicator)
• "Add to Home Screen" prompt on supported mobile browsers
• App icon and splash screen configured for iOS and Android
• Online/offline status indicator in the UI