Added a dedicated dark-mode logo variant for the platform header.

The full logo now switches to the dark-mode asset when the app is using a dark background.

The existing light-mode logo remains unchanged for light mode.

The compact mobile logo behavior remains unchanged.

Open the platform in dark mode.

The header logo now uses the higher-contrast dark-mode version automatically.

The MojaAI logo is easier to see against dark backgrounds.

This improves the polish and readability of the main app shell in dark mode.