Mobile: Push notification scaffolding

[krusty-agent]

Summary

Adds scaffolding for native push notifications using Capacitor's plugin.

Changes

• ✅ Installed @capacitor/push-notifications plugin
• ✅ Created src/lib/pushNotifications.ts service with:
  • Permission request handling
  • APNs/FCM registration
  • Push notification listeners (foreground, background, tap)
  • Token management setup
• ✅ Initialized push notifications in App.tsx after user authentication
• ✅ Added TODO comment in Settings UI for native push toggle integration
• ✅ Created comprehensive documentation in mobile/PUSH_NOTIFICATIONS.md

Documentation Includes

• iOS setup (APNs key/certificate in Apple Developer Portal)
• Android setup (Firebase project + google-services.json)
• Backend integration guide (Convex functions for token storage and push sending)
• Testing instructions (physical device for iOS, emulator for Android)
• Debugging tips and common issues

Next Steps (Not in this PR)

1. Configure Apple Developer Portal with APNs key
2. Set up Firebase project and add google-services.json
3. Create Convex mutations to store push tokens
4. Create Convex actions to send pushes via APNs/FCM
5. Test on physical devices/emulators
6. Add UI toggle in Settings for native push (optional)

Testing

Build verified: npm run build ✅

Note: Push notifications require platform-specific setup before testing:

• iOS: Physical device only (simulator doesn't support pushes)
• Android: Emulator with Play Services or physical device

See mobile/PUSH_NOTIFICATIONS.md for complete testing guide.