Notifications of reactions to messages

### Your use case

#### What would you like to do?

Receive notifications for reactions to messages.

As I said on https://github.com/element-hq/element-meta/issues/2927#issuecomment-3146431713:

> Would be nice if it could be toggled per-chat/room or could be configured to default to enabled for one-to-one conversations with anyone who is marked as a `Favourite`. It's something I use frequently to respond/acknowledge as it's something people tend to get notified for on the likes of Facebook Messenger, so can often miss acknowledgements/confirmations from people due to this.

#### Why would you like to do it?

Recreated the issue as a feature request as per https://github.com/element-hq/element-meta/issues/2927#issuecomment-3150033035

#### How would you like to achieve it?

A toggle as mentioned in https://github.com/element-hq/element-meta/issues/2927#issuecomment-3130662894

### Have you considered any alternatives?

Element is king of Matrix clients. :- )

### Additional context

_No response_

No files, tests, or client entry points are named. Start by reading the linked discussion and tracing how reaction events and notification preferences are handled in the relevant Element client; done means reactions can notify users and the requested per-chat or favourite-contact toggle has clear, tested behaviour.

Written by the indexing model from the issue text.