frontend: Add missing icon in Light Theme

[shiina424]

Description

This PR adds the missing icons that were not included in the following commit:
Audio Mixer horizontal and vertical icons, as well as the headphone icons, in the Light theme: a42a6c4

Before (horizontal and vertical icons)

before-icon.mp4

After (horizontal and vertical icons)

after-icon.mp4

Before (headphone icon)
The icons are displayed in white.

before-headphone.mp4

After (headphone icon)

after-headphone.mp4

This is the best I could do with my current knowledge.
If you see a better approach, please feel free to update or close this PR.

Motivation and Context

The Light theme was missing the horizontal and vertical layout icons, as well as the headphone icons.

How Has This Been Tested?

Tested on Windows 11 25H2 using a local build.

Types of changes

• Tweak (non-breaking change to improve existing functionality)

Checklist:

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

[Warchamp7]

Please adjust the fill color to #202020 to match the other Light icons.

[shiina424]

Please adjust the fill color to #202020 to match the other Light icons.

Adjusted the fill color.