[app] visible sync/download queue progress

[legoktm]

Description

We want to have some visible indicator when sync is running, there's a download queue or there are pending events to be sent to the server.

A few notes on the workflow:

1. sync runs, fetching the list of items and sources and files/messages
2. that list of messages populates the download queue, which then immediately starts downloading the individual messages and decrypting them
3. as the journalist interacts with the sources, they queue a list of pending events (item_seen, starring, deletion, reply). in most cases we will submit the pending events to the server ASAP but in offline mode a queue will build up.
4. sync runs upon login and then on a 60-second timer. In most cases when sync runs, nothing will happen!

[elioqoshi]

This is the sync indicator in Thunderbird at the bottom for reference

Screen.Recording.2025-11-18.at.16.04.12.mov

[legoktm]

One thing to flag from today's discussion is that we want to track whether messages that haven't been decrypted yet are queued or not (failed?), so that way for the ones that are queued we can display a React skeleton. Also Elio noted that it would be nice if we could prioritize the downloading of whichever messages the journalist clicks on.