F14: Push notifications — reminders on iOS and Android

## What to build

Implement push notifications so users are reminded about due chores. Notifications should be configurable per chore: at due time, on a repeating interval, or after an overdue threshold. Requires platform support on iOS and Android.

This is a feasibility and architecture slice: establish the notification infrastructure, permission flows, and at least one working notification type end-to-end.

## Acceptance criteria

- [ ] User can grant/deny notification permission from within the app
- [ ] At least one notification trigger works end-to-end (e.g. "notify at due time")
- [ ] Per-chore notification settings (on/off, trigger type) are configurable
- [ ] Notifications deep-link back to the relevant chore on tap
- [ ] Tested on both iOS and Android
- [ ] Notification payload respects user's chosen reminder mode (due time / interval / overdue)

## Blocked by

None — can start immediately