gui: Set Style to Fusion for all systems

[joshanne]

There was something uncanny with the windows build.

You'll notice the windows before image looks different to the Linux before (I haven't added Linux before since it looks the same as Linux after).

The fusion style is platform agnostic, and should be same across systems. See: https://doc.qt.io/qt-6/qtquickcontrols-fusion.html

Windows before:

Windows after:

Linux after:

[joshanne]

@Huibean if you have an opportunity, are you able to test the Mac build to ensure the style looks consistent between your Mac build and the Linux/windows images above. Would be very appreciated.

[Huibean]

left side is with "Fusion", I have no perference on which style is better, but since it did fix the slider style, so LGTM

[joshanne]

Personally I found the new Qt6 theme a little jarring in windows. Linux was largely unchanged.

I think by setting the theme, we're being consistent across all operating systems now.

[joshanne]

Merging this to keep consistency across the OS'