UI Scale option?

### Feature Description

Allow changing the UI scale of DMS, not just the font size

### Use Case

DMS in general appears to be smaller than applications themselves, so I would like a way to make DMS a little bigger without just changing the scale of everything else.

I know I can change the font scale, however that would make icons off-scale from the font size and things just look off. There are also issues with text wrapping and clipping that should probably be a separate issue.

### Compositor(s)

- [ ] All compositors
- [x] Niri
- [ ] Hyprland
- [ ] MangoWC (dwl)
- [ ] Sway
- [ ] Other (specify below)

### Proposed Solution

A UI scale option

### Alternatives/Existing Solutions

_No response_

No files, tests, or entry points are named in the issue. Start by tracing the existing font-scale option for the Niri configuration, then determine the UI-scale behavior and verify that DMS can become larger without changing system-wide font or icon proportions.

Written by the indexing model from the issue text.