feat(wallet): add install banner and push notifications for incoming payments

[divysam]

Summary

Closes Miracle656#171

Implements PWA install prompt and push notifications for incoming Stellar payments.

Changes

frontend/wallet/public/sw.js (new)

• Custom service worker with background Horizon polling every 30 seconds
• Fires showNotification() for each new incoming payment/create_account operation
• Notification tap deep-links to /dashboard?tx=<hash>
• Handles server-sent push events for future push subscription support
• Listens for VEIL_REGISTER_ACCOUNT message from the page to know which account to poll

frontend/wallet/app/dashboard/page.tsx

• Registers /sw.js and posts VEIL_REGISTER_ACCOUNT once wallet address is known
• Requests Notification.requestPermission() once after first successful data load (guarded by veil_notif_asked localStorage key)
• Handles ?tx=<hash> deep-link from notification tap — opens TxDetailSheet for the matching transaction

frontend/wallet/app/InstallBanner.tsx (pre-existing)

• Already implemented and wired into app/layout.tsx
• Shows on Android (uses beforeinstallprompt) and iOS (manual Share → Add to Home Screen instructions)
• Dismissal stored in localStorage with 7-day cooldown; session-level suppression via sessionStorage

Acceptance criteria

• Install banner appears on Android Chrome / iOS Safari before install
• Banner doesn't reappear after dismissal (7-day cooldown)
• Notifications fire for incoming testnet payments (SW polls Horizon every 30s)
• Tapping a notification opens the relevant tx detail (/dashboard?tx=<hash>)
• Graceful degradation when permission is denied (requestPermission catch is silent)