feat: better layout for the message list

This is to close #8 issue.
The new interface is much more "material" and ergonomic.
The sender is displayed (either compact or with full address) below.
Then, a quick & small email summary is shown.
The thread message count is only displayed if there is more than a single message.
I'm using light font here, and the same color for both unread & read messages. Unread messages are using bold font.