Implement notifications

# Problem
We have a notifications section in the the bottom navigation, but the notifications activity and design isn't ready yet.

## Potential fix
1. Design a screen for displaying notifications that the user might get. This might include things like appointment confirmations, appointment reminders etc.
2. Implement pop-up notifications and display them inside the app in the notifications section.

Feel free to trigger the notifications in any way you like.

Start by locating the existing notifications section and bottom-navigation entry, then inspect how the app currently handles appointment-related events. Done means a designed notifications screen, in-app pop-up notifications, and visible notification entries for confirmations or reminders, but the issue names no files or tests.

Written by the indexing model from the issue text.