[dev-v5][NavCategory] Fix font-family and sizing

[MarvinKlein1508]

Pull Request

📖 Description

This PR inherits the font from FluentNav for FluentNavCategory. Previously categories were using a different fon-family and font-size.

Before:

After:

The effect was even stronger on mobile devices.

Before:

After:

✅ Checklist

General

• I have added tests for my changes.
• I have tested my changes.
• I have updated the project documentation to reflect my changes.
• I have read the CONTRIBUTING documentation and followed the standards for this project.

Component-specific

• I have added a new component
• I have added Unit Tests for my new component
• I have modified an existing component
• I have validated the Unit Tests for an existing component

⏭ Next Steps

[Copilot]

Pull request overview

This pull request fixes a visual inconsistency in the Nav component set by ensuring FluentNavCategory uses the same font-family and font-size as its surrounding FluentNav content (notably important because the category renders as a native <button>, which can otherwise use UA-default font styling).

Changes:

• Update FluentNavCategory styling so .fluent-navcategoryitem inherits font-size and font-family.