Show speed during Video Studio route replays

[dakennguyen]

Feature request

Add an optional speed readout to rendered Video Studio route replay videos. It would show the recorded or calculated speed at the current point in the journey as the marker moves, using the selected kilometres or miles units.

Why

Speed adds useful context to a replay of a bike ride, drive, or run. Video Studio currently shows average speed in its settings summary, but the exported video shows distance and time without a changing speed readout. The map replay already displays speed.

Suggested behavior

• Offer a toggle to show or hide speed in the video overlay.
• Keep the displayed speed aligned with the marker and recorded time, and respect the selected km/h or mph units.
• Handle missing or unreliable speed data gracefully, for example by showing a dash rather than a misleading value.

Tested on 1.17.0; the 1.20.0 Video Studio overlay also appears not to include speed based on the source code.