Support showing error radius on map positions

This is an usual feature in many map-based applications.

The idea is that one uses the input from a second GPS (or the HDOP data) to place a circle around the given position, which translates to the area were this position could actually be.

The issue does not name a file, test, or entry point. Start by locating the map-position rendering entry point and determine how a second GPS or HDOP value is exposed; clarify the expected radius calculation and map behavior before implementation. Done means the position displays a circle representing its possible error area, with coverage for the chosen input source.

Written by the indexing model from the issue text.