[sync] UI feedback when the testbed connection is disconnected

[yoursunny]

There is limited UI feedback when the testbed connection is disconnected.

There is a "disconnect" message on the top-right corner.
However, it's easily missed if it happens when I'm away from the computer.
Afterwards, what I wrote would not be uploaded if I close the webpage at that moment.

I have to look at Connection page to determine whether I'm still connected.
There should be an icon to indicate connectivity status (reference: Visual Studio Code has a connection symbol in bottom-left corner).