Prevent duplicate instance launches and surface starting/terminating states in UI

[D-VR]

• Add a non-blocking player lock when creating instances so a second POST immediately returns 409 "already starting or running" instead of hanging.
• Persist a "starting..." state in the challenge view (survives refresh) to disable the launch button and show meaningful feedback while provisioning.
• Detect instances that have expired but still exist (cron cleanup pending) by checking since + until < now, show "terminating..." and disable launch until cleanup finishes (i.e. cleanup finishes when the instance no longer exists, i.e. 404).

Issues: Addresses #231 and improves user-facing errors for the scenarios discussed in #225, this closes #232

[NicoFgrx]

Hi, I tried to integrate the PR, but it’s not working (or maybe I don’t understand what it’s supposed to do).
From what I gather, when I start creating an instance, the "Starting..." button should remain visible even after closing the pop-up, but that’s not happening.

Any idea what might be causing this?
P.S. I synced the branch with your repo to include the latest upstream main project changes.