Add a correct "Recording start time"(?) to Profile Info

#3457 should rename "Recording started" to "Process started" or similar.
Now, we may still want to show something like "Recording started".

I think the frontend has enough information already (`meta.startTime` being the process start time, and being able to find the earliest visible sample/marker to display) to compute the local time corresponding to the left-most side of full range.
Though I don't think the name "Recording started" would be 100% correct, since it's possible some of the early recording was lost if the profile buffer got full. Maybe "Profiling range start" would work? Better suggestions welcome.

After that, Firefox _could_ store the timestamp when the profiler was started (backend change required, please let me know if needed), and store it in a new field, e.g. `meta.recordingStart`. And this could be shown correctly as "Recording started".
But I believe that we should still show "Profiling start range", especially if it's different.

Bonus idea: Show the local time corresponding to displayed profile-relative times (in seconds) in some places. E.g.: When hovering the band with the times at the top of the timeline area; When hovering a "Start" time in the Marker Table; Etc.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-421)

Start by locating the Profile Info UI and its use of meta.startTime, then inspect how the earliest visible sample or marker defines the full-range left edge. Done means the UI distinguishes the profiling-range start from any true recording start and displays the agreed local-time wording; the optional Firefox metadata and hover enhancements are separate follow-up scope.

Written by the indexing model from the issue text.