feat(desktop): modernize desktop UI with minimalist zinc palette

[iotserver24]

This PR updates the Desktop React UI to be more sleek, minimalist, and flat, taking inspiration from Claude Desktop and Cursor.

It replaces the heavy dark colors with a more modern Zinc-based palette (#09090b for the main background and #18181b for surfaces). It also flattens the chat input and message bubble rendering to remove excess borders, gradients, and shadows, creating a cleaner chat experience.

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PR created automatically by Jules for task 2951738989987908235 started by @iotserver24

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