frontend: Add minimum width to spinboxes

[Warchamp7]

Description

Adds a minimum width for spinboxes to the stylesheet to work around QTBUG-141916

Fixes #12476

Motivation and Context

Text in many spinboxes gets clipped due to a bug in how Qt calculates the widget size when the up/down buttons have their size changed by the stylesheet.

How Has This Been Tested?

👁

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• [x] My code has been run through clang-format.
• [x] I have read the contributing document.
• [x] My code is not on the master branch.
• [x] The code has been tested.
• [x] All commit messages are properly formatted and commits squashed where appropriate.
• [x] I have included updates to all appropriate documentation.