feat(notifications): implement real-time on-chain sale notifications (#9)

[Simultech369]

Summary of Changes

This pull request resolves #9 by implementing an on-chain event-driven notification subsystem for producers in crate-frontend, complete with Soroban RPC event polling, cursor persistence, navbar dropdown indicators, and real-time sale toasts.

🎯 Key Implementations

1. Soroban Event Poller & Notification State Hook (src/hooks/useNotifications.ts)

   • Implements useNotifications hook polling Soroban RPC getEvents for purchase_license and withdraw_earnings topics.
   • Persists the last processed ledger cursor in localStorage (crate_events_cursor_${address}).
   • Scopes notification records and read/unread state per connected wallet address (crate_notifications_${address}).
   • Gracefully no-ops when contract ID or wallet connection is absent.

2. Navbar Notification Dropdown (src/components/NotificationDropdown.tsx)

   • Bell icon button with dynamic unread count badge.
   • Dropdown panel showing relative time ago (Just now, 5m ago, 2d ago), sample title, amount in XLM, unread indicators, "Mark read" and "Clear" actions.

3. In-App Toast Alerts

   • Automatically surfaces in-app toast notifications with audio/visual flair when a new license purchase is detected on-chain while connected.

4. Testing & Validation (src/__tests__/notifications.test.ts)

   • Unit tests covering per-wallet storage isolation, read/unread state toggles, and cursor advancing.

---

🧪 Verification & Testing

• npx vitest run: