Fixes and cleanup before full release

[LumePart]

This PR includes small fixes and cleanup before the full UI release.

Changes:

• [backend] Read UI variables to config struct (instead of reading directly from os)
• [UI] Disable wizard system select if it is predefined in .env
• [UI] Restrict defining sleep to API systems (plex, jellyfin, emby)
  • Subsonic uses a dedicated API endpoint for scan status, which is implemented in Explo.

Some bugs not in this PR yet:

• Refreshing the main page while having most values defined in .env brings the user back to Wizard step 1
• Background cover art fetching blocks the whole UI from loading
• The cover art should be fetched asynchronously from the UI
  • I will also optimize and limit this in the backend