[layouts] Use nicer number formatting for dynamic map scale labels

[nyalldawson]

Fixes #51764

[uclaros]

@nyalldawson why does the scale need rounding though, while the user explicitly sets a round scale number?

[nyalldawson]

@uclaros

Cos of floating point precision -- without it you'll get scales like 29999.99999999999

[uclaros]

Cos of floating point precision -- without it you'll get scales like 29999.99999999999

Well, what is not that obvious to me is why the scale is calculated when the user explicitly sets 30000, which should be nicely representable by doubles. And furthermore, why it is not rounded upon calculation (the variable map_scale is still 29999.99999999999) and only rounded on the dynamic label?

[nyalldawson]

@uclaros

That's a very good question! For reasons which predate my time in qgis (ie > 15 years) map items have always used extent+map dimensions as the canonical property from which all other properties (including scale and center) are derived. So entering a specific fixed scale doesn't actually store that scale -- it recalculates the visible extent and stores that, and re-derives the scale from that.

I... don't like.... this approach.

I would love to see an enum setting allowing the center + scale to be the canonical map property, with the extent derived from that. And then this new setting becoming the default going forward....