Friends: invite notifications

### Description

We would like to prompt users when they revisit [Notangles](https://notangles.devsoc.app) about friend invites that are waiting for them to manage.

### Requirements

- Need number of **incoming** friend invites labelled in a notification circle
- if friends sidebar list collapsed: label is shown on friends toggle button
- else if add friends modal not open: label is shown on "Add Friends" button
- label is always shown on "Friend requests" button
- Accepting/Deleting an incoming invite will reduce the number in the notification
- Notification is not shown if there are no incoming invites