Starting a new transcription silently discards the one in progress

What happened?

There is no confirmation and no queue when starting a second transcription. If a transcription is already running, starting a new one aborts it immediately and discards its result. Nothing is saved, nothing is recoverable, and no warning is shown.

This is easy to trigger by accident, and on a long video it can throw away ten or more minutes of finished work.

Suggested fix

Any of these would remove the data loss, in increasing order of effort:

1. Show a confirmation dialog: "A transcription is in progress. Cancelling it will discard the result. Continue?"
2. Disable the start control while a transcription is running, so the situation cannot occur.
3. Queue the new request behind the running one. Option 3 overlaps with a separate feature request I am filing about queuing multiple URLs, but options 1 and 2 are much smaller changes and would fix the data loss on their own.

Steps to reproduce

Steps to reproduce

1. Paste a video URL and start the transcription.
2. Wait until the download reaches 100% and transcription begins.
3. Start a new transcription with a different URL.
4. The first transcription stops. Its result is gone from the UI, and the app begins downloading the new video. Expected: either the new item is queued behind the current one, or the app asks for confirmation before discarding work in progress.

Actual: the running transcription is cancelled and its output is lost without any prompt.

What OS are you seeing the problem on?

MacOS

Relevant log output