Implement install queue

Installing/updating many mods in parallel seems to occasionally cause weird errors. While the backend cause is written stateless and hence should be fully parallelisable some of the OS operations around adding/removing files seem to cause weird interference.

The easiest solution would be to only allow one mod install process to run at once. That would mean blocking all mod installs until the first one is done.

For the user that's a bit annoying to deal with to have to wait for one mod install to be done before being able to initiate the next one so the next best solution would be to add every mod install to a queue and then have a worker dispatch elements from the queue until empty.

The queue management + worker would be done in frontend to keep backend stateless.

Start by tracing the frontend mod-install entry point and the existing install/update flow; the issue names no files or tests. Done means install requests are queued, processed one at a time until the queue is empty, and no longer interfere with one another.