feat(mobile): local notification scheduler for hunt expiry (#219)

[euniceamoni]

Summary

Implements issue #219 — schedules a local OS notification 1 hour before a joined hunt expires, fully offline using expo-notifications.

Changes

• mobile/package.json — added expo-notifications ~0.30.0
• mobile/utils/huntNotifications.ts — new utility with scheduleHuntExpiryNotification and cancelHuntExpiryNotification
• mobile/app/_layout.tsx — registers notification handler and requests OS permission on launch
• mobile/store/huntStore.ts — adds joinHunt(huntId) which schedules the expiry reminder after a player joins

Usage

import { joinHunt } from '@store/huntStore';

// after successful on-chain registration:
await joinHunt(hunt.id);

Notes

• Notification fires completely offline — no network call involved
• Delivered even when the app is backgrounded or closed
• Duplicate reminders are prevented (existing notification cancelled before rescheduling)
• No-ops silently if endTime is missing or already within 1 hour closes [Mobile] Local Notification Scheduler #219