Improve info on sync status

[almarklein]

To avoid self-hosters from losing data, as in #443:

• Make it more obvious that the sync icon is clickable (with a hoover shadow, tooltip, or cursor shape change). (The sync icon does stand out, but it's not obvious that it's clickable.)
• In the info, explain more about how the local caching works, vs syncing with the server. Or perhaps explain that in detail in a webpage/docs and link to it from the info box.
• Perhaps a warning (and link to the above doc) in the docs that explain setting up the server.