style: revamp desktop UI to use flat zinc dark theme

[iotserver24]

This PR revamps the React desktop UI to follow a minimalist, flat, and grayscale dark-mode aesthetic.

• Updates the global color palette to rely heavily on Tailwind's neutral zinc colors (zinc-950 for background, zinc-900 for surfaces, etc.).
• Simplifies the ChatPanel text input component by removing the large border radiuses and background gradients in favor of a clean, flatter aesthetic.
• Cleans up MessageBubble styling to provide subtle markdown rendering, notably stripping heavy borders and backgrounds from pre and code blocks for a sleeker look.

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PR created automatically by Jules for task 881096415902681041 started by @iotserver24

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