Display zoom level map scale (e.g. 1:10,000) beneath scale bar when changing zoom

Summary

As a geologist digitising lines via Mergin Input, I would like to know the absolute scale that I am working at so that I can digitise with an appropriate level of detail.

Description

Current behaviour

When are user zooms in or out in the Mergin Input user interface, a scale bar is shown that demonstrates how long a 50 m line (or whatever is appropriate) would be. After a while, the scale bar disappears again.

The example below is taken from the Mergin documentation.

Sometimes, when users are digitising lines, they need to know the scale that they are working at so that they can mark vertices with an appropriate level of detail. If they are using a 1:10k or 1:25k Ordnance Survey basemap, the scale is easy to determine. However, if they are using a DEM or hillshade, this is not possible.

Enhancement

Our geologists have requested an enhancement to the current scale bar implementation, which is to add another line beneath the scale bar that reports the zoom level e.g. 1:10,000. This way they could zoom to a level that is appropriate.

They would not be able to hit a precisely defined level by pinching their fingers e.g. 1:10,124. Rendering the zoom levels to just 2 significant figures would be good enough e.g. 1:150, 1:1500, 1:15k, 1:150k, 1:1.50M.

I don't know if 1:100,000 or 1:100k is easier to read or understand.

Start by locating the existing scale-bar implementation in the Mergin Input mobile UI and trace how it updates when the map zoom changes. Confirm how map scale is calculated and formatted, then add a second line beneath the scale bar showing a rounded ratio; done means it appears at the requested zoom levels without disrupting the existing scale bar.