Export network requests as HAR

I'd like to be able to get the requests in the HAR format. Either a button to download from the network panel (example https://share.firefox.dev/4eEUs8H), or some kind of URL convention where tacking `.har` onto the end of a share link loads the JSON for the active thread instead of rendering the profiler UI (example https://share.firefox.dev/4eEUs8H.har)

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-796)

Start by reviewing the network panel and share-link handling in the profiler UI, as described in the issue. Determine which of the two requested export paths is intended, then verify that the completed work produces HAR-format JSON for the active thread and does not render the profiler UI for the URL-based path.

Written by the indexing model from the issue text.