This PR hardcodes the font size for the OS notification and adjusts the
copywriting on the `DoneStreaming` scenario.

1. Reason for the former change is because notifications always have a
fixed width and height, so any responsive design strategy here wouldn't
fully work.
2. Reason for the latter is because when the assistant response is done
streaming, that _can_ mean "changes have been applied" (previous label)
but it can also not mean that. So, I'm making it more generic now.

Release Notes:

- N/A