Fractional Scale under 100%

### Problem

Sometimes there's a situation where one wants to make the UI *smaller* than 100%, like on a relatively large, but low-resolution screen (some laptops I've used had this)

### Proposal

elementary/gala should allow setting the display scale to less than 100%, to shrink down the UI

### Prior Art (Optional)

KDE Plasma supports this in their scaling options, by just setting the scale to a percentage value like `70%` it will scale down the UI.

Begin in elementary/gala by locating its current display-scaling entry point and how percentage values are validated; the issue provides no file or test path. Compare the behavior with KDE Plasma’s 70% scaling example, then confirm that a sub-100% setting can be selected and consistently shrinks the UI.

Written by the indexing model from the issue text.