Add internal FocusSyncer component

[sgwilym]

Recently I noticed that, when reopening a tab with a react-earthstar app, workspace data would be missing new documents that were created since putting the tab in the background. This is especially noticeable on a phone.

What I've added here is an internal component which kicks off a manual sync for all workspaces when a tab is refocused.

This only happens if EarthstarPeer's isLive state is true.

[cinnamon-bun]

Aha, yes, this is a conceptual bug with the sync algorithm.

Currently, live sync opens a steam of new events from the server. It should open a stream of events since a given time, so it can recover from gaps. The server-sent events spec has a way to handle this, Last-Event-Id, which we're not using yet.

For now the only way to recover from gaps is to do a full sync again.

This PR is a reasonable fix until the core sync algorithm is fixed, I support it 👍

Various thoughts:

• visibilitychange and focus events happen frequently with every tab change in a browser, I think? We don't need to do a full sync then since the server-sent event stream should remain connected, we only need to do it when the connection is dropped. So this will increase load on pubs by quite a bit, but that's ok for now.

• It would be better to detect a connection drop but that would have to happen in the core library, and anyway the Last-Event-Id is the right answer

• Another option is to run an interval timer once a second and check if the time since the last interval is more than 10 seconds, indicating a phone has gone to sleep and then woken up

• If rapidly switching tabs over and over, will this start several sync operations simultaneously? Let's see... no it's fine, the core sync code is smart about that, no problem