fix: theme handling in navbar component

[skillpoint-dev]

Expected behaviour

When providing different logos in "appIcon" and "appIconDark" the logo is switched with switching of the theme.

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Current behaviour

The "appIcon" was always return since the theme was always "undefined" and the handling of "auto" was also missing.

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Steps to reproduce/test

overlayFSPath: '/etc/ots/assets'
appIcon: 'logo.svg'
appIconDark: 'logo_dark.svg'

in the root of the repo and the 2 mentioned SVGs in the "/frontend" subdirectory

<svg height="100" width="100" xmlns="http://www.w3.org/2000/svg">
  <circle r="45" cx="50" cy="50" fill="black" />
</svg>

<svg height="100" width="100" xmlns="http://www.w3.org/2000/svg">
  <circle r="45" cx="50" cy="50" fill="white" />
</svg>

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None of this work was done using any type of LLM/AI.